Thank you for testing.
Seems like it is a graphics of some sort, I’ll try rolling back 4 versions since it has happened last three. Plus the fact that you are getting a “timed consumption of resources and the FPS drops like 20 FPS each 2-3 sec” tells me there still might be something universal there, just not as drastic on your hardware as mine.
So far I have replaced the offending Particles with static mesh and rotating them with a looping Timeline, turning off all shadows/indirect lighting/etc. has my simulation apparent overhead as low as it was with Particles(fps is as high as before). That may be my fix till I can get more corroborating evidence with the particles evil or lack of it.
Bottom line, Particles no prob 4.18, Particles big prob 4.19 - which means there is something going on Epic changed regardless of it affecting my machine(and likely others with similar) only as far as anyone has reported yet.