Hybrid drives don't always work well. The one in my iMac did, until I reinstalled macOS and then restored from a Time Machine backup, and now I get hard disc speeds, with a second or two delay to (say) open a small file, rather than instantly as with an SSD, as was the case before. It's been a week or two, and I don't know why Fusion Drive didn't optimise the data placement to move frequently used files to the SSD.