Speeds - I bet Eclipse is unbeaten in the world of IDEs. All of them. Incremental compiler that IBM brought long time ago from VisualJ is, still, one of the best in the world of compilers. Eclipse 2.0 or 2.1 on 700MHz Celeron with only 128MB of memory (on Windows 98 - on my 2000 laptop) works almost on par with IntelliJ on 3.3GHz i7 with 16GB of memory... So, no comparison there...
Caching? No idea - too much I/O is not good for SD, but doesn't Eclipse doing very similar as well with constant recompilation why you're typing?
JDK 12? I hope so. It would be really odd if they don't as, as I said, they are the top IDE for Java development. They are always one step in front of Eclipse in many things and I can easily bet that they have at least twice as many (if not much more) Java developers than Eclipse
Also, it seems more stable and more rounded with more (current, contemporary) frameworks/tools/libraries plug-ins support.
But, I would suggest getting cheap USB3 to SATA adapter and some old, discarded, low capacity SSD of someone (something people throw away these days - for instance, who needs 3+ year old 80GB SSD?) and switch to it with Pi4. It make visible difference for little extra. (something like this for instance https://www.ebay.co.uk/itm/USB-3-0-To-C ... 3541113187