Mostly the bloat. It's not just the order of magnitude more RAM and CPU usage than any reasonable editor uses, it's the lag that is really grating.
Yes, compiling can take time, that's reasonable. What isn't reasonable is for the user interface to be slower than I am. My computer can perform tens of billions of operations per second. When I click on something, there's no technical reason it couldn't respond in at least a 30/th of a second, if not a 60th. There's plenty of software that's been around for decades that can do this just fine.