Sounds like it's having to work hard to access scraps of data which ideally should be recorded in a single stream, so yes, do a defrag.
But why use such archaic hardware? Hard discs are fast on streaming but painfully slow for random access. Install plenty of flash and you get faster random access, whilst the slight penalty for streaming remains constant rather that deteriorating.
I use a tablet with one hundred per cent flash and no nasty mechanical hard disc, so I would say that, wouldn't I?