Video jerky after adding noise reduction and rendering

I've noticed this behavior myself. I'm also running 10.4.10 in Mojave on 2017 i5 iMac. It's annoying.

From what I can figure out — it generally only happens with the frame rate of the video clip *doesn't match* the frame rate of the project (and it doesn't happen with *all* clips, so the type of original encoding may play a part in it). Would that be the case here? If so, try applying Modify > Retime > Automatic Speed to the clip.

Automatic speed just conforms a video clip to the project frame rate. For example, if you have a 25fps clip in a 30fps project, in order to make 25 frames equal the same length of time as 30 frames, 5 extra frames need to be added. Five frames get played and then the fifth frame plays twice to make 6 frames played. Automatic speed just plays all the frames once - no repeats.

When it comes to fractional frame rates (which a lot of cameras still use) the difference in speed between 29.97 and 30 is not very noticeable, but with automatic speed, FCP will not try to adjust the playback speed to match the "fractional frames". [Technically, there's no such thing as a fractional frame, but playback systems must slow down playback by 1% in order to keep time from drifting— and in digital systems, there *will be* rounding errors!]

I also suspect that some people are more sensitive to these frame gymnastics than others.

I prefer to use Better Performance routinely as playback is improved and any reduction in perceived quality during editing goes unnoticed.

Before I suggested it, I tested a clip with noise reduction added and got smooth playback under Better Performance but dropped frames when changed to Better Quality. . . .however, my 2017 iMac still has its original 8GB RAM.

Have you got the Quality set to Better Performance in the View dropdown menu?

If not, try it.