I'd hope it is what Seba and Shmee suggested, the free space. Its a 500GB SSD and it has 'only' 50GiBs of free space. But that is quite a dramatic performance drop so it might be something more serious.
I'd use CrystalDiskInfo to check it for errors.
The 'rule' I've always applied, which is what was recommended practice when I asked about such things on other forums decades ago is not to go below 10% free space, particularly on the primary system drive.
I had a 40GB 2.5" HDD in the laptop I used for years and when the free space went below 4GiB the speed started to drop noticeably. I had the same thing with 128GB SSDs when they went below 12GiB. That was one of the reasons I upgraded to the 500GB SSDs I'm using now instead.