diff --git a/client/requestUtil.js b/client/requestUtil.js
index 0ab3ed8..144fb13 100644
--- a/client/requestUtil.js
+++ b/client/requestUtil.js
@@ -3,10 +3,12 @@
 const awsSdk = require('aws-sdk')
 const cryptoUtil = require('./cryptoUtil')
 const proto = require('./constants/proto')
+const {limitConcurrency} = require('../lib/promiseHelper')
 const s3Helper = require('../lib/s3Helper')
 const serializer = require('../lib/serializer')
 
 const CONFIG = require('./config')
+const PUT_CONCURRENCY = 1000
 const S3_MAX_RETRIES = 1
 const EXPIRED_CREDENTIAL_ERRORS = [
   /The provided token has expired\./,
@@ -218,11 +220,12 @@ RequestUtil.prototype.currentRecordPrefix = function (category) {
 /**
  * Puts a single record, splitting it into multiple objects if needed.
  * @param {string} category - the category ID
- * @param {Uint8Array} record - the object content, serialized and encrypted
+ * @param {object} record - the object content
  */
 RequestUtil.prototype.put = function (category, record) {
+  const encryptedRecord = this.encrypt(record)
   const s3Prefix = this.currentRecordPrefix(category)
-  const s3Keys = s3Helper.encodeDataToS3KeyArray(s3Prefix, record)
+  const s3Keys = s3Helper.encodeDataToS3KeyArray(s3Prefix, encryptedRecord)
   return this.withRetry(() => {
     const fetchPromises = s3Keys.map((key, _i) => {
       const params = {
@@ -236,6 +239,12 @@ RequestUtil.prototype.put = function (category, record) {
   })
 }
 
+/**
+ * Like put() but with limited concurrency to avoid out of memory/connection
+ * errors (net::ERR_INSUFFICIENT_RESOURCES)
+ */
+RequestUtil.prototype.bufferedPut = limitConcurrency(RequestUtil.prototype.put, PUT_CONCURRENCY)
+
 RequestUtil.prototype.s3PostFormData = function (objectKey) {
   let formData = new FormData() // eslint-disable-line
   formData.append('key', objectKey)
diff --git a/client/sync.js b/client/sync.js
index efcdc2d..8d4f615 100644
--- a/client/sync.js
+++ b/client/sync.js
@@ -128,9 +128,11 @@ const startSync = (requester) => {
     ipc.send(messages.RESOLVED_SYNC_RECORDS, category, resolvedRecords)
   })
   ipc.on(messages.SEND_SYNC_RECORDS, (e, category, records) => {
+    logSync(`Sending ${records.length} records`)
     if (!proto.categories[category]) {
       throw new Error(`Unsupported sync category: ${category}`)
     }
+    // let puts = []
     records.forEach((record) => {
       // Workaround #17
       record.deviceId = new Uint8Array(record.deviceId)
@@ -139,8 +141,11 @@ const startSync = (requester) => {
         record.bookmark.parentFolderObjectId = new Uint8Array(record.bookmark.parentFolderObjectId)
       }
       logSync(`sending record: ${JSON.stringify(record)}`)
-      requester.put(proto.categories[category], requester.encrypt(record))
+      const bufferedPut = requester.bufferedPut(proto.categories[category], record)
+      // puts.push(bufferedPut)
     })
+    // TODO: Respond with sync complete message
+    // Promise.all(puts)
   })
   ipc.on(messages.DELETE_SYNC_USER, (e) => {
     logSync(`Deleting user!!`)
diff --git a/lib/promiseHelper.js b/lib/promiseHelper.js
new file mode 100644
index 0000000..6e327f1
--- /dev/null
+++ b/lib/promiseHelper.js
@@ -0,0 +1,42 @@
+'use strict'
+
+/**
+ * Wrap a Promise-returning function so calls to it fill a queue which has
+ * a concurrency limit.
+ * e.g. there is an API rate limited to 10 concurrent connections.
+ * const getApi = (arg) => window.fetch(arg)
+ * const throttledGetApi = limitConcurrency(getApi, 10)
+ * for (let i; i < 1000; i++) { throttledGetApi(i) }
+ * @param fn {function} Function which returns a Promise
+ * @param concurrency {number} Maximum pending/concurrent fn calls
+ * @returns {function}
+ */
+module.exports.limitConcurrency = function (fn, concurrency) {
+  var queue = null
+  var active = []
+  const enqueueFnFactory = function (_this, args) {
+    return function () {
+      const enqueued = fn.apply(_this, args)
+      enqueued.then(function () {
+        active.splice(active.indexOf(enqueued), 1)
+      })
+      active.push(enqueued)
+      return {
+        enqueued,
+        newQueue: Promise.race(active),
+      }
+    }
+  }
+  return function () {
+    var enqueueFn = enqueueFnFactory(this, arguments)
+    if (active.length < concurrency) {
+      const promises = enqueueFn()
+      queue = promises.newQueue
+      return promises.enqueued
+    } else {
+      const advanceQueue = queue.then(enqueueFn)
+      queue = advanceQueue.then(promises => promises.newQueue)
+      return advanceQueue.then(promises => promises.enqueued)
+    }
+  }
+}
diff --git a/test/promiseHelper.js b/test/promiseHelper.js
new file mode 100644
index 0000000..4346287
--- /dev/null
+++ b/test/promiseHelper.js
@@ -0,0 +1,34 @@
+const test = require('tape')
+const promiseHelper = require('../lib/promiseHelper')
+
+test('promiseHelper', (t) => {
+  t.plan(1)
+
+  t.test('limitConcurrency', (t) => {
+    t.plan(1)
+
+    t.test('calls the original function the same number of times with correct args', (t) => {
+      t.plan(2)
+      const EXPECTED_CALL_COUNT = 100
+      let callCount = 0
+      const asyncFun = (i) => new Promise((resolve, reject) => {
+        setTimeout(() => {
+          callCount += 1
+          resolve(i)
+        }, Math.round(10 * Math.random()))
+      })
+      const throttedAsyncFun = promiseHelper.limitConcurrency(asyncFun, 3)
+      const promises = []
+      let expectedSum = 0
+      for (let i = 0; i < EXPECTED_CALL_COUNT; i++) {
+        promises.push(throttedAsyncFun(i))
+        expectedSum += i
+      }
+      Promise.all(promises).then((results) => {
+        const sum = results.reduce((a, b) => a + b)
+        t.equal(callCount, EXPECTED_CALL_COUNT)
+        t.equal(sum, expectedSum)
+      })
+    })
+  })
+})