some tests (failed tests) try to use two Neo4J instances. It must be fixed.

I thought that's what you were looking for a way to fix this.

Tmpfs increases performance dramatically and has low labor intensity (about 15 minutes)

What does it have to do with what we are talking about right now?
It's not like I can change the build in maven so that it mounts tmpfs on every machine that is running the build.
Remember that we are trying to make the whole build faster without tweaking the machine underneath.

And .. 4 second for each test class ... is very small.

I'm not sure how many time we create a factory, but let's say it is 30 times, 30 * 4s = 120s = 2 minutes
2 minutes on every build is a lot of time saved