My SSD seems slow, could it be the link speed? I just replaced my old HD from a late-2009 iMac with a new Samsung SSD 850 EVO 1TB. 
It was installed by an official Apple repair store but I am worried because it seems slower than my new macboook pro (also with SSD)
I tested with Blackmagic Disk Test and it:


Writes 253 MB/s
Reads 268 Mb/s


Official specs say it should:


Write 520 MB/s
Read 500 Mb/s


And it seems from reviews they get numbers close to official ones.

I checked my SATA Chipset in the iMac and it says: "Link Speed: 3 Gigabit".

Does this mean that 3000 Mb/s (Megabits) are 375 MB/s (Megabytes) and it cannot transfer faster (even though the SSD is faster)?
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/