It depends on what kind of drive you refer to.
For all kinds of drives (HDD or SSD) speed will decrease if there's less than ~20% of free space.
SSD drives get slower by the years depending on the amount of i/o writes..
If it's an SSD enabling TRIM is recommended by this terminal command: sudo trimforce enable
This can improve i/o speed of the SSD in case it has been used a lot without TRIM.
If it's an SSD mounted in a SATA bay /optical bay, speed is limited to SATA-2(~300mb/s).
One could install an PCIe adapter to get SATA-3 speeds(~600 mb/s ) e.g. Accelsior S.
Backdraws are that the drive will be seen as external by the OS and the speed improvement is not very spectacular thus maybe not worth ~ 50,- ...
The speed of i/o is independent if its the OS disk or a data disk.