Slow down browser rendering
Converting what @z0r said in the comments to an answer:
In Chrome, open devtools and select the Performance tab 
Make sure Screenshots is checked
Press the record button (or hit Ctrl + E)
Do your activity
Stop recording
Hover over the timeline to see the screenshots of the screen as things change. 
Use the timeline or profiler on your browser inspector. Here you can see, what functions take down the speed.
The accepted answer is good; I use and recommend Chrome Dev Tools as well.
As an alternative to Chrome Dev Tools:
Several 'website performance analysis' services offer timeline views. Run some internet searches and you'll find various free and paid options.
Try webpagetest.org
It's open source, highly regarded and has been running for years. It may offer different information, or is accessible in a different way, than Chrome Dev Tools.
In the test results, click "Filmstrip View".