diff --git a/app/api/courses/route.ts b/app/api/courses/route.ts
index 7313025..5a10268 100644
--- a/app/api/courses/route.ts
+++ b/app/api/courses/route.ts
@@ -1,38 +1,56 @@
+import { Prisma } from '@prisma/client'
 import { NextRequest, NextResponse } from 'next/server'
 import { prisma } from '@/lib/prisma'
 import { getLocalThumbnailUrl } from '@/lib/thumbnail-index-server'
 
 export const dynamic = 'force-dynamic'
 
+const DEFAULT_PAGE_SIZE = 10
+const MAX_PAGE_SIZE = 100
+const SORT_FIELDS = new Set(['updatedAt', 'createdAt', 'name', 'progress'])
+
+type SortOrder = 'asc' | 'desc'
+
+type CourseWithLibraryData = Prisma.CourseGetPayload<{
+  include: {
+    _count: { select: { modules: true } }
+    progress: true
+    courseTags: { include: { tag: true } }
+  }
+}>
+
+function parsePositiveInteger(value: string | null, fallback: number): number {
+  const parsed = Number.parseInt(value ?? '', 10)
+  return Number.isFinite(parsed) && parsed > 0 ? parsed : fallback
+}
+
 export async function GET(request: NextRequest) {
+  const startedAt = performance.now()
+
   try {
     const { searchParams } = request.nextUrl
-    
-    // Pagination
-    const page = parseInt(searchParams.get('page') || '1')
-    const limit = Math.min(parseInt(searchParams.get('limit') || '10'), 100)
+    const page = parsePositiveInteger(searchParams.get('page'), 1)
+    const limit = Math.min(
+      parsePositiveInteger(searchParams.get('limit'), DEFAULT_PAGE_SIZE),
+      MAX_PAGE_SIZE
+    )
     const skip = (page - 1) * limit
-    
-    // Search
-    const search = searchParams.get('search') || ''
-    
-    // Filters
-    const filter = searchParams.get('filter') || 'all' // all, in-progress, completed, not-started, favorites, tag:<tagId>
+    const search = (searchParams.get('search') || '').trim()
+    const filter = searchParams.get('filter') || 'all'
     const tag = searchParams.get('tag') || ''
     const tagFilter = tag || (filter.startsWith('tag:') ? filter.slice(4) : '')
     const favoritesOnly = searchParams.get('favorites') === 'true'
-    const sortBy = searchParams.get('sortBy') || 'updatedAt'
-    const sortOrder = searchParams.get('sortOrder') || 'desc'
-    
-    // Build where clause
-    const where: any = { hidden: false }
+    const requestedSort = searchParams.get('sortBy') || 'updatedAt'
+    const sortBy = SORT_FIELDS.has(requestedSort) ? requestedSort : 'updatedAt'
+    const sortOrder: SortOrder = searchParams.get('sortOrder') === 'asc' ? 'asc' : 'desc'
+
+    const where: Prisma.CourseWhereInput = { hidden: false }
 
     if (search) {
-      const term = search.trim()
       where.OR = [
-        { name: { contains: term } },
-        { displayName: { contains: term } },
-        { slug: { contains: term } },
+        { name: { contains: search } },
+        { displayName: { contains: search } },
+        { slug: { contains: search } },
       ]
     }
 
@@ -58,96 +76,120 @@ export async function GET(request: NextRequest) {
       }
     }
 
-    // Build orderBy. Favorites are pinned above non-favorites for library organization.
-    const secondaryOrderBy: any = {}
-    if (sortBy === 'name') {
-      secondaryOrderBy.name = sortOrder
-    } else if (sortBy === 'progress') {
-      secondaryOrderBy.updatedAt = sortOrder
-    } else {
-      secondaryOrderBy[sortBy] = sortOrder
-    }
-    const orderBy: any[] = [{ favorited: 'desc' }, secondaryOrderBy]
+    const secondaryOrderBy: Prisma.CourseOrderByWithRelationInput =
+      sortBy === 'name'
+        ? { name: sortOrder }
+        : { updatedAt: sortOrder }
 
-    const total = await prisma.course.count({ where })
+    const orderBy: Prisma.CourseOrderByWithRelationInput[] = [
+      { favorited: 'desc' },
+      secondaryOrderBy,
+    ]
 
-    const courses = await prisma.course.findMany({
-      where,
-      orderBy,
-      skip: Math.max(0, skip),
-      take: limit,
-      include: {
-        _count: { select: { modules: true } },
-        progress: {
-          where: { userId: 'local-user', lessonId: null },
+    const databaseStartedAt = performance.now()
+    const [total, courses, tags] = await Promise.all([
+      prisma.course.count({ where }),
+      prisma.course.findMany({
+        where,
+        orderBy,
+        skip: Math.max(0, skip),
+        take: limit,
+        include: {
+          _count: { select: { modules: true } },
+          progress: {
+            where: { userId: 'local-user', lessonId: null, moduleId: null },
+            orderBy: { lastWatched: 'desc' },
+            take: 1,
+          },
+          courseTags: {
+            include: { tag: true },
+            orderBy: { tag: { name: 'asc' } },
+          },
         },
-        courseTags: {
-          include: { tag: true },
-          orderBy: { tag: { name: 'asc' } },
-        },
-      },
-    })
+      }),
+      prisma.tag.findMany({
+        orderBy: { name: 'asc' },
+        include: { _count: { select: { courseTags: true } } },
+      }),
+    ])
+    const initialDatabaseDuration = performance.now() - databaseStartedAt
 
-    const courseIds = courses.map(c => c.id)
-    const lessonCounts = await prisma.lesson.groupBy({
-      by: ['moduleId'],
-      where: { module: { courseId: { in: courseIds } } },
-      _count: true,
-    })
+    const courseIds = courses.map((course) => course.id)
+    const enrichStartedAt = performance.now()
 
-    const moduleToCourse = new Map<string, string>()
-    const modules = await prisma.module.findMany({
-      where: { courseId: { in: courseIds } },
-      select: { id: true, courseId: true },
-    })
-    modules.forEach(m => moduleToCourse.set(m.id, m.courseId))
+    const [modulesWithCounts, completedProgress] = courseIds.length > 0
+      ? await Promise.all([
+          prisma.module.findMany({
+            where: { courseId: { in: courseIds } },
+            select: {
+              courseId: true,
+              _count: { select: { lessons: true } },
+            },
+          }),
+          prisma.progress.groupBy({
+            by: ['courseId'],
+            where: {
+              userId: 'local-user',
+              courseId: { in: courseIds },
+              lessonId: { not: null },
+              completed: true,
+            },
+            _count: { _all: true },
+          }),
+        ])
+      : [[], []]
 
-    const moduleLessonCounts = new Map<string, number>()
-    lessonCounts.forEach(lc => {
-      const courseId = moduleToCourse.get(lc.moduleId as string)
-      if (courseId) {
-        moduleLessonCounts.set(courseId, (moduleLessonCounts.get(courseId) || 0) + lc._count)
-      }
-    })
-
-    const coursesWithProgress = await Promise.all(courses.map(async (course) => {
-      const courseProgress = course.progress[0]
-      const totalLessons = moduleLessonCounts.get(course.id) || 0
-      let completedLessons = 0
-      let percentage = 0
-      let lastWatched = null
-
-      if (courseProgress) {
-        completedLessons = courseProgress.completed ? totalLessons : 0
-        percentage = courseProgress.completed ? 100 : 0
-        lastWatched = courseProgress.lastWatched.toISOString()
-      }
-
-      return {
-        ...course,
-        _count: { ...course._count, lessons: totalLessons },
-        progress: { completedLessons, totalLessons, percentage, lastWatched },
-        thumbnail: await getLocalThumbnailUrl(course.slug) ?? null,
-        description: course.description,
-        tags: course.courseTags.map((connection) => connection.tag),
-      }
-    }))
-
-    if (sortBy === 'progress') {
-      coursesWithProgress.sort((a, b) =>
-        sortOrder === 'asc'
-          ? a.progress.percentage - b.progress.percentage
-          : b.progress.percentage - a.progress.percentage
+    const lessonCountByCourse = new Map<string, number>()
+    for (const module of modulesWithCounts) {
+      lessonCountByCourse.set(
+        module.courseId,
+        (lessonCountByCourse.get(module.courseId) || 0) + module._count.lessons
       )
     }
 
-    const totalPages = Math.ceil(total / limit)
-    const tags = await prisma.tag.findMany({
-      orderBy: { name: 'asc' },
-      include: { _count: { select: { courseTags: true } } },
-    })
+    const completedCountByCourse = new Map<string, number>(
+      completedProgress.map((entry) => [entry.courseId, Number(entry._count._all)])
+    )
 
-    return NextResponse.json({
+    const coursesWithProgress = await Promise.all(
+      courses.map(async (course: CourseWithLibraryData) => {
+        const totalLessons = lessonCountByCourse.get(course.id) || 0
+        const completedLessons = Math.min(
+          completedCountByCourse.get(course.id) || 0,
+          totalLessons
+        )
+        const percentage = totalLessons > 0
+          ? Math.round((completedLessons / totalLessons) * 1000) / 10
+          : 0
+        const summaryProgress = course.progress[0]
+        const thumbnail = course.thumbnail || await getLocalThumbnailUrl(course.slug)
+        const { courseTags, progress: _progressRows, ...courseData } = course
+
+        return {
+          ...courseData,
+          _count: { ...course._count, lessons: totalLessons },
+          progress: {
+            completedLessons,
+            totalLessons,
+            percentage,
+            lastWatched: summaryProgress?.lastWatched.toISOString() ?? null,
+          },
+          thumbnail: thumbnail ?? null,
+          tags: courseTags.map((connection) => connection.tag),
+        }
+      })
+    )
+
+    if (sortBy === 'progress') {
+      coursesWithProgress.sort((a, b) => {
+        const difference = a.progress.percentage - b.progress.percentage
+        return sortOrder === 'asc' ? difference : -difference
+      })
+    }
+
+    const enrichDuration = performance.now() - enrichStartedAt
+    const totalPages = Math.ceil(total / limit)
+    const response = NextResponse.json({
       courses: coursesWithProgress,
       tags,
       pagination: {
@@ -159,6 +201,18 @@ export async function GET(request: NextRequest) {
         hasPrev: page > 1,
       },
     })
+
+    response.headers.set(
+      'Server-Timing',
+      [
+        `db;dur=${initialDatabaseDuration.toFixed(1)}`,
+        `enrich;dur=${enrichDuration.toFixed(1)}`,
+        `total;dur=${(performance.now() - startedAt).toFixed(1)}`,
+      ].join(', ')
+    )
+    response.headers.set('Cache-Control', 'private, no-store')
+
+    return response
   } catch (error) {
     console.error('Courses API error:', error)
     return NextResponse.json(
@@ -166,4 +220,4 @@ export async function GET(request: NextRequest) {
       { status: 500 }
     )
   }
-}
\ No newline at end of file
+}