My understanding of this phenomenon is that it affects the longevity of the drive more than it affects the performance, at least from what an end user observes.
SSD media can only write to empty file pages, but they can only erase a file block (collection of pages, normally around 128). Without TRIM (which the OS uses to tell the drive which pages and blocks it can safely erase), the SSD needs to move pages around in order to free up blocks in order to write new data.
What this boils down to is the SSD needs to perform multiple physical writes to do one logical write of the data sent by the OS. This is a phenomenon called Write Amplification.
The longevity issue comes into play when you consider that SSDs have a limited number of write/erase cycles per cell (1,000-100,000 per cell depending on the media). This is mitigated somewhat by wear leveling which is an automatic use of the least-used cells on a drive to avoid uneven wear, but write amplification limits how much wear leveling can take place. Wear leveling also leads to some write amplification on its own (due to needing to move data which is not changing under certain scenarios).
Since there are still no moving parts in the SSD, it will obviously be much faster than a normal drive even with these issues. However, the relative speed to a non-affected drive could be many times slower depending on how much write amplification is taking place.