I had reported a similar behavior about two weeks ago (Rules no longer executing after upgrade to OH 4 - #4 by ldm). Thanks, @mstormi, for pointing me here.
As I stated in that other thread, I got these problems after restoring a --full backup from OH 3 into OH 4.3. I later tried using a backup that was made without the --full option, and since then 4.3 has been running fine here, with hundreds of rules. Depending on how you got to your current installation, it might be worth to create a backup (without --full), do a clean/purged re-installation (again), restore the backup and see what that brings?
The two backups I experimented with were taken back-to-back, just a few seconds apart, from the same source installation, one with --full, the other one without. So that is definitely what made the difference in my case.
For completeness: My installation is working now (at least for the time being). It’s a relatively large setup with close to 1000 items and almost 10k lines of rules in DSL code. I do not see excessive CPU usage, but I do see the huge memory footprint mentioned in this thread. In my case it’s 8.4G of virtual process size (on a 64 bit Ubuntu x86 machine, Zulu 17 JRE).