I doubt IntelliJ would become much faster if it were written in C++. The bottleneck on all of my machines running it has always been I/O. If it wasn't I/O, it was waiting for other tools (gradle, maven, node, take your pick) to complete one of their build steps.

It's not an application that's meant to be light weight like VS Code, I believe Jetbrains Fleet is much more suitable for that.

Maybe there are caching advantages to be gained, but those can be implemented in Java as well. I think the most important optimizations wouldn't stand to gain much from a rewrite into native code.