diff --git a/services/web/frontend/js/serviceWorker.js b/services/web/frontend/js/serviceWorker.js
index 8ee1c37409..18123e6e5e 100644
--- a/services/web/frontend/js/serviceWorker.js
+++ b/services/web/frontend/js/serviceWorker.js
@@ -3,6 +3,25 @@ const MIN_CHUNK_SIZE = 128 * 1024
 
 const PDF_FILES = new Map()
 
+const METRICS = {
+  epoch: Date.now(),
+  cachedBytes: 0,
+  fetchedBytes: 0,
+  requestedBytes: 0,
+}
+
+/**
+ *
+ * @param {number} size
+ * @param {number} cachedBytes
+ * @param {number} fetchedBytes
+ */
+function trackStats({ size, cachedBytes, fetchedBytes }) {
+  METRICS.cachedBytes += cachedBytes
+  METRICS.fetchedBytes += fetchedBytes
+  METRICS.requestedBytes += size
+}
+
 /**
  * @param {FetchEvent} event
  */
@@ -18,6 +37,7 @@ function onFetch(event) {
   if (ctx) {
     return processPdfRequest(event, ctx)
   }
+
   // other request, ignore
 }
 
@@ -27,8 +47,9 @@ function processCompileRequest(event) {
       if (response.status !== 200) return response
 
       return response.json().then(body => {
-        handleCompileResponse(body)
-        // The response body is consumed, serialize it again.
+        handleCompileResponse(response, body)
+        // Send the service workers metrics to the frontend.
+        body.serviceWorkerMetrics = METRICS
         return new Response(JSON.stringify(body), response)
       })
     })
@@ -41,8 +62,12 @@ function processCompileRequest(event) {
  * @param {Object} file
  * @param {string} clsiServerId
  * @param {string} compileGroup
+ * @param {Date} pdfCreatedAt
  */
-function processPdfRequest(event, { file, clsiServerId, compileGroup }) {
+function processPdfRequest(
+  event,
+  { file, clsiServerId, compileGroup, pdfCreatedAt }
+) {
   if (!event.request.headers.has('Range') && file.size > MIN_CHUNK_SIZE) {
     // skip probe request
     const headers = new Headers()
@@ -96,6 +121,8 @@ function processPdfRequest(event, { file, clsiServerId, compileGroup }) {
       })
     )
   const size = end - start
+  let cachedBytes = 0
+  let fetchedBytes = 0
   const reAssembledBlob = new Uint8Array(size)
   event.respondWith(
     Promise.all(
@@ -109,6 +136,22 @@ function processPdfRequest(event, { file, clsiServerId, compileGroup }) {
                 }`
               )
             }
+            const blobFetchDate = getServerTime(response)
+            const blobSize = getResponseSize(response)
+            if (blobFetchDate && blobSize) {
+              const chunkSize =
+                Math.min(end, chunk.end) - Math.max(start, chunk.start)
+              // Example: 2MB PDF, 1MB image, 128KB PDF.js chunk.
+              //     | pdf.js chunk |
+              //   | A BIG IMAGE BLOB |
+              // |     THE     FULL     PDF     |
+              if (blobFetchDate < pdfCreatedAt) {
+                cachedBytes += chunkSize
+              } else {
+                // Blobs are fetched in bulk.
+                fetchedBytes += blobSize
+              }
+            }
             return response.arrayBuffer()
           })
           .then(arrayBuffer => {
@@ -134,6 +177,8 @@ function processPdfRequest(event, { file, clsiServerId, compileGroup }) {
           const insertPosition = Math.max(chunk.start - start, 0)
           reAssembledBlob.set(new Uint8Array(arrayBuffer), insertPosition)
         })
+
+        trackStats({ size, cachedBytes, fetchedBytes })
         return new Response(reAssembledBlob, {
           status: 206,
           headers: {
@@ -152,16 +197,40 @@ function processPdfRequest(event, { file, clsiServerId, compileGroup }) {
 }
 
 /**
+ *
+ * @param {Response} response
+ */
+function getServerTime(response) {
+  const raw = response.headers.get('Date')
+  if (!raw) return undefined
+  return new Date(raw)
+}
+
+/**
+ *
+ * @param {Response} response
+ */
+function getResponseSize(response) {
+  const raw = response.headers.get('Content-Length')
+  if (!raw) return 0
+  return parseInt(raw, 10)
+}
+
+/**
+ * @param {Response} response
  * @param {Object} body
  */
-function handleCompileResponse(body) {
+function handleCompileResponse(response, body) {
   if (!body || body.status !== 'success') return
 
+  const pdfCreatedAt = getServerTime(response)
+
   for (const file of body.outputFiles) {
     if (file.path !== 'output.pdf') continue // not the pdf used for rendering
     if (file.ranges) {
       const { clsiServerId, compileGroup } = body
       PDF_FILES.set(file.url, {
+        pdfCreatedAt,
         file,
         clsiServerId,
         compileGroup,