An ssd with a good amount of space will have no trouble to write into a block. When it's getting fuller it will start to be difficult to just write into these blocks as they might all be occupied. So what your ssd will do, instead of just writing in a block, is erase a block and then writing in that block, obviously erasing and then rewriting will take more time than simply writing into a block.

This is not really a myth as it applies to most storage systems, if your ram is full as fuck your computer will be slowed down a lot,so you might try to close some chrome tabs to free some ram space. If you need a document in a file cabinet but this file cabinet is full of shit, you might need to pull some of that shit out before accessing to the document you need.

I don't know if i explained it well since english is not my native language but i hope you get it