HDDs should also have about 10-> 15% free so that defrag can do it's work. However, HDDs performance will depend on where the data is located on the platters. If the data to be read (or written) is close to the center of the disk, performance takes a big hit as the angular velocity is MUCH lower than for data written to the out edges of the platters.
Note HDDs have an Average access speed, typically around 10->13 mSec. SSDs is 0.1 or less.
SSDs as others have states, and like HDDs, should have a Min of 10->15% left free - Just for different reason. While you do Not run defrag on an SSD, they do employ; Wear leveling, Garbage collection (Which trim improves this function) and for them to work their magic, they need this free space.
While a HDD that is say 70 used you will notice a performance hit - NO SO with a 70% filled SSD. SSDs do not have angular velocity, nor outer inner areas.
I experienced huge problems when I downloaded all my photos (74GB) to SSD (221GB) with windows installed on it. It took half an hour to start my computer after multiple attempts and I could not even start it in safe mode, and programs were responding extremely slow. SSD came to 200GB out of 221, so it was 10% free space. My laptop came back to normal after I erased all my photos again. Conclusion: Full SSD is definitely affecting computers performance. QED