Not arguing with what was previously said.
But my impression was that the cache in a cached HD was mainly to improve boot up time.
My SSD made such a big change in
everything I ran on my PC that at this point the difference between cached & uncached feels insignificant (though I still use them for back ups).
About your issue -
How full is your drive? (not an issue on SSD drives)
What's your fragmentation look like?
If your drive is fairly full then new additions may end up being badly fragmented.
Also is the cached drive a 5400 or 7200 rpm?
For a while (maybe still? haven't checked in the last 2 years) Seagate was putting out some 5400 cached drives where the rpm wasn't on the label.
At one point I was using one of these as my sample drive and chucked it for straight 7200.