Intellij does that a little bit of incremental compilation. They half integrated the eclipse Java compiler at some point but they never really made using it a thing. Most projects just hand off compilatio to Gradle or maven, which means you lose a lot of time waiting for that and dealing with various caches in intellij and gradle/maven getting out of sync with each other. The process for fixing that generally involves boiling the oceans and running the CPU at 400% for a few minutes.
Eclipse compiles two orders of magnitudes faster when you have it properly setup. I'm not exaggerating. It's really good at this. Intellij users have never seen this level of performance. I've tried to explain it a few times to users that insisted they had intellij configured just right. The minimum execution for a unit test after a 1 character code change is the key benchmark for this. Run a test, change something trivial, run it again. If that takes more than half a second and has a noticeable delay, it's not fast. Eclipse user to be closer to 50-100ms. Even on a (now) ten year old laptop. With intellij, it's closer to 3-5 seconds. They have improved things over the years but it never got even close to that level of performance.
