NetBeans is unusably slow

I used to use NetBeans 8 on JDK8, it was tolerable, but slow.

Now I have a new laptop, I've tried nb8, nb10 on jdk8, jdk11, with g1gc, cmsgc, other gc. They are all UNUSABLY slow. By which I mean, it takes over a minute just to open a file.

Mostly this is because things are waiting for things waiting for very inefficiently implemnted file I/O, editor parsing, or things like that. I've watched a 12-core Xeon chip spend over 4 hours just attempting to handle the Active Reference Queue Daemon thread.

The slowdown is somewhat incremental - I can escape it as long as I'm happy to restart NetBeans every hour or so (which I'm not happy to do).

Attached a stack dump to get you started, this took over a minute to open a 284-line Java file from idle.

Please feel free to ask all the obvious questions, although I'm fairly sure all the obvious things have been done. Mostly, I've got GC down to a negligible amount.

If there's one or more features of NetBeans I can disable to get acceptable behaviour while still being able to realistically edit Java code, please tell me and I will disable them because I am at the end of my rope after using (and developing for) NetBeans for nearly 10 years. If I can get IntelliJ to load a Java/Gradle project neatly before this gets fixed, I'm gone.