I’m posting this comment by K P as an answer, because K P does not have enough reputation.
It just needed some more memory to prevent repeated garbage collection. I found the file idea.vmoptions [aka idea64.exe.vmoptions] and increased the memory for InteiiJ to run (Xms = 512m and Xmx = 2048). The CPU usage has come down to 0.2 – 10% when nothing is being done on it.