garbage collection.
a secure erase is good for piece of mind, but the first generation ssd's had no GC, so they suffered a lot without a secure erase, now days things are very different, especially if a decent amount of over provisioning.
I own 2 samsung 830s (128gig)
and now in my main rig a lovely 850 pro 512gig.
One of the 830s is in a pc I am using for testing, and I have done multiple OS reinstalls only quick formatting, no secure erase. It still bench's at proper speeds.