I'm using WebStorm 2020.1 EAP on my LG UltraFine 5K and it works well, and I have done so for at least a year. Connected via TB3, 2016 MacBook.

One thing that could help is killing Docker, everything becomes more responsive -- on Mac that is, given that it's literally a VM running on Hyperkit. That's especially the case when you have a big project on in Jetbrains IDEs (CLion, WebStorm, IntelliJ, ...).