I've noticed the same performance issues these last couple of months. From what I have seen, when the slowdown occurs any action within the site seems to be affected (i.e. add card, remove card, autocomplete, hover card preview, deck listing, inventory listing/searching to name a few). On average, actions take around 3-5 seconds. On bad days I've seen it upwards of 15-20 seconds per action, at which point I just postpone deck building until it improves. Unfortunately, it tends to fluctuate throughout the day.
fwiw, here is my environment:
Chrome: Version 78.0.3904.108
OS: Mac 10.14.5
Download speed: ~150 Mbps