As someone with decades of software engineering under my belt, my opinion is they are the best IDEs out there by a comfortable margin. It's possible the issue you're encountering when accidentally opening them is due to index updates which would occur disproportionately often on startup for people who otherwise never use them. Other than that, performance is more than satisfactory 90+% of the time in my experience.
That's what I would have said about Eclipse about 15 years ago.
I rarely have performance issues with Jetbrains IDEs, and even rarer still that they bog down the whole machine. Get a properly specced computer (32+GB RAM, 10+ core chip) if you don't already have one. Also try adjusting memory settings - https://www.jetbrains.com/help/idea/increasing-memory-heap.h...
That's just it. In my company there are basically three sets of developers. The very experienced ones prefer vim/emacs. The experienced ones tend to like these more established IDEs, and the younger developers absolutely hate these IDEs and find them to be slow and bloated.
I can't really comment on whether they are good or not since from my perspective I see people who are productive and unproductive using both of these tools, so to me it just looks like mostly a matter of preference. But newer developers don't seem to like these established IDEs and see them as you said: big, slow and laggy.
Intellij is an IDE where VS Code is a bit mroe then a text editor.
When you open a project for the first time it takes time because it is indexed , maybe it will index your dependencies if you did not bother to disable that.
IWhen i open a file /project from a collegue that uses VS Code is filled with errors and warnings because their VS code text editor is not actually understanding the code they are editing.
People that say this are completely out of touch. You know there are VSCode extensions for Java (made by Oracle and Redhat themselves), right? Once you install that you get Intellisense, debugging, etc. Same with C#, Python, etc.
I know that, once but once you install that you will not be able to praise VS Code performance anymore, you just get an inferior Intellij IDE in all dimensions.
It seems like JetBrains may be giving up on Fleet. Honestly though, I think their established IDEs are good enough to be worth a few bugs and performance issues.
I have PTSD from accidentally opening CLion or PyCharm. Fans starts spinning and there is dozens of seconds wait to close this thing down.