There are ways to improve it.
1. use default theme only
2. upvote my suggestion construct3.ideas.aha.io/ideas/C3-I-1231
Editor needs extra grouping element. So you can hide events and action to improve performance and look
3. Find Css bug, report Github, then report Crbugs and in months you get it fixed :)
I do use the default theme since I'm someone who doesn't care if my tools look pretty, I only care about their raw capabilities and power. My friend said something along the lines of your suggestion hypothetically could improve C3 GUI lag in much larger projects, but I don't know enough about browser technology to say one way or the other despite I do know things like C# but I find C3 makes me able to do 10x more in the same amount of time as if I actually wrote programming for just one thing and if you optimize your game enough then you can do even more than Unity from that I've found where I had like 800 enemies on screen in one of my games that use line of sight and test every single tile they are on for terrain type while also testing every single character constantly that is on screen to see if their team IDs match or not to determine if they should attack each other and that was only when the game started to become to slow in FPS, but this is definitely A++ acceptable performance because my game is 320x240 resolution but I would never even need 100 enemies on screen at once even if the resolution was 1024x768 lol. Even if I've complained a lot about C3 at times, I have stuck with it for about 3 years now despite having only a subscription payment option I hated at first because it has always shown a lot of potential when it already is really good compared to some of the most popular IDEs.