diff --git a/spec/integ/matrix-client-crypto.spec.js b/spec/integ/matrix-client-crypto.spec.js
index 966385394..cf6f33c52 100644
--- a/spec/integ/matrix-client-crypto.spec.js
+++ b/spec/integ/matrix-client-crypto.spec.js
@@ -130,7 +130,12 @@ function expectAliClaimKeys() {
         return {one_time_keys: result};
     });
 
-    return aliTestClient.httpBackend.flush("/keys/claim", 1);
+    // it can take a while to process the key query, so give it some extra
+    // time, and make sure the claim actually happens tather than ploughing on
+    // confusingly.
+    return aliTestClient.httpBackend.flush("/keys/claim", 1, 20).then((r) => {
+        expect(r).toEqual(1);
+    });
 }
 
 
@@ -263,16 +268,16 @@ function sendMessage(client) {
 
 function expectSendMessageRequest(httpBackend) {
     const path = "/send/m.room.encrypted/";
-    let sent;
+    const deferred = q.defer();
     httpBackend.when("PUT", path).respond(200, function(path, content) {
-        sent = content;
+        deferred.resolve(content);
         return {
             event_id: "asdfgh",
         };
     });
-    return httpBackend.flush(path, 1).then(function() {
-        return sent;
-    });
+
+    // it can take a while to process the key query, so give it 20ms
+    return httpBackend.flush(path, 1, 20).then(() => deferred.promise);
 }
 
 function aliRecvMessage() {
diff --git a/spec/mock-request.js b/spec/mock-request.js
index bf1a1b04e..e4a929a61 100644
--- a/spec/mock-request.js
+++ b/spec/mock-request.js
@@ -36,15 +36,23 @@ HttpBackend.prototype = {
      * Respond to all of the requests (flush the queue).
      * @param {string} path The path to flush (optional) default: all.
      * @param {integer} numToFlush The number of things to flush (optional), default: all.
-     * @return {Promise} resolved when there is nothing left to flush.
+     * @param {integer=} waitTime  The time (in ms) to wait for a request to happen.
+     *    default: 5
+     *
+     * @return {Promise} resolves when there is nothing left to flush, with the
+     *    number of requests flushed
      */
-    flush: function(path, numToFlush) {
+    flush: function(path, numToFlush, waitTime) {
         const defer = q.defer();
         const self = this;
         let flushed = 0;
         let triedWaiting = false;
+        if (waitTime === undefined) {
+            waitTime = 5;
+        }
         console.log(
-            "HTTP backend flushing... (path=%s  numToFlush=%s)", path, numToFlush,
+            "HTTP backend flushing... (path=%s numToFlush=%s waitTime=%s)",
+            path, numToFlush, waitTime,
         );
         const tryFlush = function() {
             // if there's more real requests and more expected requests, flush 'em.
@@ -57,7 +65,7 @@ HttpBackend.prototype = {
                 flushed += 1;
                 if (numToFlush && flushed === numToFlush) {
                     console.log("  Flushed assigned amount: %s", numToFlush);
-                    defer.resolve();
+                    defer.resolve(flushed);
                 } else {
                     console.log("  flushed. Trying for more.");
                     setTimeout(tryFlush, 0);
@@ -65,11 +73,11 @@ HttpBackend.prototype = {
             } else if (flushed === 0 && !triedWaiting) {
                 // we may not have made the request yet, wait a generous amount of
                 // time before giving up.
-                setTimeout(tryFlush, 5);
+                setTimeout(tryFlush, waitTime);
                 triedWaiting = true;
             } else {
                 console.log("  no more flushes.");
-                defer.resolve();
+                defer.resolve(flushed);
             }
         };
 
diff --git a/src/crypto/DeviceList.js b/src/crypto/DeviceList.js
index 49b4a4179..6cb7735e6 100644
--- a/src/crypto/DeviceList.js
+++ b/src/crypto/DeviceList.js
@@ -306,41 +306,57 @@ export default class DeviceList {
         ).then((res) => {
             const dk = res.device_keys || {};
 
+            // do each user in a separate promise, to avoid wedging the CPU
+            // (https://github.com/vector-im/riot-web/issues/3158)
+            //
+            // of course we ought to do this in a web worker or similar, but
+            // this serves as an easy solution for now.
+            let prom = q();
             for (const userId of downloadUsers) {
-                console.log('got keys for ' + userId + ':', dk[userId]);
-
-                // map from deviceid -> deviceinfo for this user
-                const userStore = {};
-                const devs = this._sessionStore.getEndToEndDevicesForUser(userId);
-                if (devs) {
-                    Object.keys(devs).forEach((deviceId) => {
-                        const d = DeviceInfo.fromStorage(devs[deviceId], deviceId);
-                        userStore[deviceId] = d;
-                    });
-                }
-
-                _updateStoredDeviceKeysForUser(
-                    this._olmDevice, userId, userStore, dk[userId] || {},
-                );
-
-                // update the session store
-                const storage = {};
-                Object.keys(userStore).forEach((deviceId) => {
-                    storage[deviceId] = userStore[deviceId].toStorage();
+                prom = prom.delay(5).then(() => {
+                    this._processQueryResponseForUser(userId, dk[userId]);
                 });
-
-                this._sessionStore.storeEndToEndDevicesForUser(
-                    userId, storage,
-                );
-
-                if (token) {
-                    this._sessionStore.storeEndToEndDeviceSyncToken(token);
-                }
             }
+
+            return prom;
+        }).then(() => {
+            if (token) {
+                this._sessionStore.storeEndToEndDeviceSyncToken(token);
+            }
+            console.log('Completed key download for ' + downloadUsers);
         });
     }
+
+    _processQueryResponseForUser(userId, response) {
+        console.log('got keys for ' + userId + ':', response);
+
+        // map from deviceid -> deviceinfo for this user
+        const userStore = {};
+        const devs = this._sessionStore.getEndToEndDevicesForUser(userId);
+        if (devs) {
+            Object.keys(devs).forEach((deviceId) => {
+                const d = DeviceInfo.fromStorage(devs[deviceId], deviceId);
+                userStore[deviceId] = d;
+            });
+        }
+
+        _updateStoredDeviceKeysForUser(
+            this._olmDevice, userId, userStore, response || {},
+        );
+
+        // update the session store
+        const storage = {};
+        Object.keys(userStore).forEach((deviceId) => {
+            storage[deviceId] = userStore[deviceId].toStorage();
+        });
+
+        this._sessionStore.storeEndToEndDevicesForUser(
+            userId, storage,
+        );
+    }
 }
 
+
 function _updateStoredDeviceKeysForUser(_olmDevice, userId, userStore,
         userResult) {
     let updated = false;