Can defragmenting an SSD just once cause a loss of performance, or is that something that occurs only if done regularly?
The short answer is this: you don't have to defrag your SSD.
To understand why, we need to look at the purpose of defragmenting. Defragging ensures that large files are stored in one continuous area of a hard disc driveso that it can be read in one go. Mechanical drives have a relatively long seek time of approximately 15ms, so every time a file is fragmented you lose 15ms finding the next one, And this really adds up when reading lots of different files split into lots of different fragments.
However, this isn't an issue with SSDs, because the seek time is about 0.1ms. You aren't really going to notice the benefit of defragged files--which means defragging has no performance advantages with an SSD.
An SSD moves data that's already happily on your disk to other places on your disk, often sticking it at a temporary position first. Thats's what gives defragmenting a disadvantage for SSD users You're writing data you already have, which uses up some of the NAND's limited rewrite capability, -- with no performance advantage to be gained from it.
So basically, don't defrag your drive because at best it won't do anything, at worst it does nothing for your performance and you will use up write cycles doing it. Having done it a few times isn't going to cause you much trouble, but you don't want this to be a scheduled, weekly type thing.