Some further tests have shown, that everything is fine as long as I start only 16 clients. My CPU has 16 cores and 32 threads. After starting 2 or 3 more clients CPU goes up to 100%.
So it seems if you go start more clients than you have real CPU cores you spend a lot of time in context switches. Kernel system time is quite low, around 10% to 15%, so it seems that is not on the windows side, maybe more on the Java side.
I also have a lag problem with bot manager and stealth mode. When I run 6 accounts on my machine with bot manager, CPU usage is around 10%. When I run 25 accounts on my machine with bot manager, CPU is always at 100% (unless the bots take a break) and the bot clients lag a bit. Settings are fps 15. Low CPU mode doesn't really make a difference. The CPU has 16 cores and 32 threads.
RAM is not a problem (used around 50% of available RAM). Disk is a M2 SSD and is idling. GPU is at 20% according to windows task manager.
PM me if you need a log or more information