This SSD below had seen some use. The red line for writes was its performance as I began testing it: very poor performance dropping to a stunningly low 30 MB/sec.
Some SSDs benefit, and some do not but at least in this case, performance has held up since, at about 6X the speed of the impaired starting behavior.
Generally speaking, read speeds are unaffected and remain high, but it is write speeds that suffer when an SSD is ailing. And in general, one should be cautious about making regular backups when an SSD starts to behave like this.
The graph shows writing and reading the entire drive capacity (MB/sec, 1000 large files to fill), before and after recondition.