My SSD seems slow, could it be the link speed?
I would make sure that TRIM is enabled. Without TRIM, SSDs will run slower. I use Trim Enabler to turn on TRIM on my Samsung SSDs.

https://www.cindori.org/software/trimenabler/

Nowadays it can enable TRIM without disabling kext signing.

https://www.cindori.org/safely-enable-trim-on-yosemite-and-el-capitan/