In Chrome (and I'm guessing other browsers) you can go into the network tab and slow down networking. I don't think there is a similar option for CPU perf.

There are several race conditions in many webpages related to waiting for video or images to load.

Tangentially my late 2018 i5 MBA feels sluggish compared to my 2014 MBP. Simple things like switching windows are noticeably slower and janky. Unfortunately my 2014 MBP has an no longer supported NVidia GPU by Chrome/Firefox/Photoshop so I may have to upgrade but I know it will basically feel no different than what I have except I'll have the touch bar which I'm not really interested in.