I always have at least 15% of empty storage on both ssds.
That may not be enough.
I tried to make more empty space and it is the same..
Have you tried disabling pagefile, hibernation, and system restore?
No I did not.
Actually aI have no idea what pagefile is, but after some googling I have discovered, that it will not help. But thanks for tip!
Test your SSD's with a tool like CrystalDiskMark and let us see the results. Some of the early EVO small drives had lousy performance. Have you turned off indexing on your drives? How many processes does the Task Manager/Performance window show running?
Whatever you do, do not buy a hard disk drive to try to solve your problem.