if you have not enabled TRIM, you now know why proponents of TRIM suggest that it is not optional. When you do not TRIM unused blocks, your drive eventually fills up with deleted data and Writes slow to a crawl.
Use this Terminal command in 10.10.4 or later:
sudo trimforce enable
... then enter your login password (which will not be echoed). It will read you the Riot Act, ask you to confirm this is what you want, and do a Restart. When it has restarted, future deletions will delete unused data blocks on the drive. But it will be no faster. To make it faster, you need to get rid of a lot of deleted data.
The way to do that is to Restart in Safe mode, which honors the setting of trimforce. During its Disk check, it will TRIM all the unused blocks. Then your drive will be measurably faster.