Note: This is my own opinion, and not that of my employer (Google) or based on trade secrets or other IP from my employer.
I don't know if this is actually true in SSDs that are actually used, but there is a real cost/write-performance tradeoff. You can make writes faster by using SLC rather than MLC and by having a higher proportion of physical capacity to reported capacity in order to reduce write amplification. Looking into the future, there could be a journal in an even faster kind of memory like PCM or memristors. Still, the controller can only do so much to hide the properties of the chip, and if your drive is only flash, then the trend in the recent past for the chips is that reads are getting faster and writes are getting slower (based on the move towards smaller process sizes and more levels).
I don't know if this is actually true in SSDs that are actually used, but there is a real cost/write-performance tradeoff. You can make writes faster by using SLC rather than MLC and by having a higher proportion of physical capacity to reported capacity in order to reduce write amplification. Looking into the future, there could be a journal in an even faster kind of memory like PCM or memristors. Still, the controller can only do so much to hide the properties of the chip, and if your drive is only flash, then the trend in the recent past for the chips is that reads are getting faster and writes are getting slower (based on the move towards smaller process sizes and more levels).