Happens to me as well with many cores. The only way to keep it at 100 % for a longer time is to put a tuf value for initial population . However after 1 day I starts to drops and I have to re start.
I believe this slowdown is related to bad memory usage and long run of garbage collector. We made a lot of improvements in build 108, now it uses less memory and should be more effective.
I let the build run for 2 days without any slowdown.