Ive been using a HDD 1tb drive as a scratch disk for PS. Would a Samsung 850 evo 250mb perform better than 1tb WD 7200rpm drive.
Think of it this way: You have a small desk in your living room (or study or bedroom) and a large table in the dinning room. You are working on a lot of documents (maybe you're a lawyer or office worker) or artwork (maybe you're an artist), but those papers almost cover your small desk. So what do you do?
In most cases, people tend to take papers they are not working on at the moment, and place them on the big table (ie: dinning table), while they work on the papers on their desk. Every time they needed "that" paper, they would take some papers they just finished to the big table, put it there, pick up the papers they need, take to desk.
Scratch disk operation works similar.
Your memory (RAM) is the small desk, your hard drive (either of the HDD or SSD will do) is the big table. If your memory is full, Adobe software will swap inactive data (ie what you are not currently working on) to the HDD/SSD, and bring the active data you are working on into the RAM.
Therefore changing from a 1TB HDD drive down to a 250MB SSD is like changing the big table for a smaller table. Now you have less room for any papers you are working on.
You need to maximise your RAM. Find out how much RAM your motherboard can support, and go for maximum RAM. That would be like swapping a small desk for a bigger oak desk. ie: If your RAM is 12GB, but motherboard can support up to 32GB, then consider going for 32GB.
(However that would work assuming you're using 64-Bit Windows not 32-Bit Windows.)
Second option is if upgrading HDD or SSD, go higher not lower. In other words instead of 1TB, aim for 2TB or 3TB, not cutting down to 250MB. (That part is like swapping your family sized dinning table for a very long boardroom table, more space for more paperwork.)
While it is possible that swapping HDD for SSD as scratch disk may help due to SSD being faster, but work is still faster if it was still in the RAM. It is the size of RAM, not speed of transfer that is the important key point. The less you rely on scratch disk, and the more you rely on RAM, the work in PS would not be slowed down.
Try to think about the option of getting more RAM first, and if not possible then consider getting SSD as second option.
If possible, keep your 1TB HDD, but add the SSD as another drive, and set up your PS so it use the SSD instead of HDD as scratch disk (leaving the 1TB for files you saved), and it use as much of the 250GB as possible.