Let's try make a report here with make and models of the SSDs affected by boot and r/w slow downs.
I have a 256Gb 840 EVO.
HS w/ supplemental update and trim enabled takes 47 seconds from chime to desktop (auto login).
I will add the time later with TRIM disabled. Macmini late 2014 2.6Ghz.
Ok, I'm adding system details here, but I'm not convinced the issue is SSD make/model related. I'll explain why below:
MacBook Pro 13" Mid 2010. 4GB RAM with a Sandisk Ultra II 120GB SSD. Tried both High Sierra 10.13.3 and 10.13.4. Also tried with 10.13.3 previously clean installed on a different machine on a Kingston SSDnow 300 V 60GB SSD. Also tried with a clean install of High Sierra 10.13.4 on a Kingston SSDnow 200 V+.
Cold boot times started off at 1:20 (m:ss) and pretty much stayed consistently there across all variations with a couple of exceptions. That's from pressing the power button to sitting at a usable desktop after inputting the logon password. When the machine booted slowly, app launch times were extended beyond reasonable/tolerable levels too - e.g. 1 minute to launch the App Store, 12 seconds to launch Mail with no accounts set up. 10 seconds to launch Photos with no photos in the library.
I didn't try all combinations of all variables but stopped when performance was acceptable.
My observations can be summarised as follows:
The first clean install of 10.13.4 on the Kingston 200 V reduced boot times to 32secs. That's with trimforce disabled (the default). After enabling trimforce, boot times jumped up to 1 minute.
By now I'd already tried trimforce on and off on the original Sandisk Ultra II with 10.13.4. This was an in place upgrade from 10.13.3 which in turn was an in place upgrade from something earlier by someone else. The FS had already been updated to APFS by the original OS update.
Something made me carry out a PRAM reset. The same install with the Sandisk now cold booted in 39 seconds. That's with Trimforce enabled. App Store launched in 11 secs.
So I went for a clean install of 10.13.4 on the Sandisk. Cold boots now take 32 secs (incl inputting the password). App Store launches in 4 secs. Photos in 2secs. Everything else feel nice and snappy as it ought to.
I would add that a) I created an APFS volume for the clean install using Disk Utility. b) Those timings are with trimforce enabled.