SSD 4k read speed reduced after october update (win10 v1809)

i am using transcend ssd220s 240 GB ssd in my acer laptop(2015 with sata 3 6 gbps) (windows 10 v1803) initially i had tested the drive with crystal disk mark only 4k read/write speed .....i got 27-28 MB/s

but after 4-5 months of use and with windows 10 (v1809) ..i tested it again but this time 4k read speed is 22-23 MB/s

ssd health is at 99% i have two partition ...for windows c drive size is 99.9 GB with 53 GB occupied and the other partition is almost completely empty........(123 GB free)
can anyone tell me what could be the cause?...is the windows 10 1809 build.....is someone having the same problem...?

You won't notice the difference in real use, it is just a synthetic benchmark.
The first time you may have run that just after an optimisation.

As it is getting on for 5 years old that maybe a factor, drives don't last forever.