It uses most of the CPU power because to be able to offer refactoring services the IDE needs to maintain a indexed representation (database) of the source code. Because of that indexing work, even in large codebases you can call up all the call sites of a method or many other things and the results appear in a fraction of a second. If the IDE had to read every source file to do that every time, refactoring support would be impractical.

The flip side of that is that most of the work the IDE does is understanding the source code as it changes and maintaining those indexes, including indexes of all the dependencies. Delphi just did not even try to do that work, and Delphi apps typically had few dependencies if any anyway.

IntelliJ being slow isn't my experience. But I know it can get very slow in some cases, like if it's running out of RAM. Make sure to respond to notifications if it says it's running low on memory.

VSCode can do it faster. It's an electron app talking to exteral processes over a socket. No excuses!

Zed opens instantly and can give autocompletion instantly even for C++ projects.

But this thread starts with the article complaining that those external processes have to be restarted every twenty minutes. Exaggerating, I'm sure. But a few days ago I helped a colleague who was using both IntelliJ and VS Code on his laptop. The whole machine has dragged to a halt, a kind of slowdown I never see on Macs normally. A quick trip to the Activity Monitor showed that VS Code processes had been in a spin loop for days. Somehow they were trashing the performance of the entire machine.

The ironic thing is, I came over to take a look because my colleague had been complaining IntelliJ was slow. Kill off the errant Electron processes and suddenly IntelliJ was fast.

At any rate, startup time isn't the be all and end all for an IDE. I only (re)start my IDE when it needs an upgrade.