Transfer speeds bottom out

Hi guys,

So this seems to be a constant issue plaguing my system. When I transfer across a bunch of mp4 files it transfers at 450 MB/s for the first 10 secondsish then drops down to 40 after that. I'm transferring these files from an SSD to HDD. Any idea what's going on? I could understand if this were a lot of small files but it seems odd considering the entirety of the folder is large video files.

looks like cache of HDD,
because it's full the speed gets less.