Stutter after applying optical flow

Hello everyone, 

I have an 2020 Macbook Pro with the M1 chip. I slow down clips a lot in my editing and for that i use optical flow. But i noticed that when i use optical flow and even if i slow it down just a couple percent it stutters when i rendered it. In some cases it brings a cool effect but not in my current project. The footage is 1080p 50fps so i dont think that would be the main problem. 

Does anyone have an explanation? 

Kind Regards, 

JRV