I have a 2015 MacBook for work and use jet brains. It’s quite snappy and responsive. Id expect it to be better on newer hardware but am left wondering why performance degraded so much for people?
But it's in no way any worse on modern machines that what you see in your 2015 MacBook (I have several Macs and can attest). People complain despite getting better performance than what you get on later Macs/PCs.
The other case is too many / too slow plugins. Not all are made equal. But as far as the core functionality, there has been no regression all the years I'm using it.
I highly suspect the performance problems some people experience are due to plugins. When I was doing some Elixir/Phoenix work, the Elixir plugin made it unusable sometimes. Clearly that plugin had some serious issues which would appear at times. I have not had the same performance problems in Rubymine or in vanilla IDEA.
Or who knows; could be something about the projects - number or sizes of files, language used and references between files, etc.