iMac 27" Late 2015

+1 on EtreCheck.

EtreCheck will test and show drive transfer speeds; Apple Diagnostic will not. Those speeds, especially how Read and Write speeds compare, can really help with diagnosis in SSD matters like this.

Also, what SSD did you use—make and model? Not all "quality branded" SSDs are Mac friendly. We know that the OWC products work, and that the Crucial MX series of 2.5-inch SATA 6GB drive is very good (avoid the Crucial BX- series).

The Etrecheck report will also show if TRIMforce is enabled on the drive. That can be an issue. I replaced the old mech HDD in my 2012 MacBook Pro with OWC's Electra 6G SSD in 2017. It worked a trick for three years, then the write speed dropped by half and continued to plummet until the computer was writing no faster the ti was with the mech HDD. OWC had recommended not enabling TRIM so I did not whe I initially installed it.

At the advice of our amazing contributor here, Grant Bennet-Alder, I enabled TRIM. It was not until after numerous safe boots that the drive returned to its former speed. However, It is still running on the nominals ( Write/Read about 500MB/sec) nearly two years later.

So you can try a safe boot, but let the computer sit idle in safe boot for and hour or so before a normal restart and test. Apparently some housekeeping is involved that takes time.

To help us get a better picture of your iMacs and their apps and any possible log summaries, run EtreCheck and post the reports here using the "Additional Text" icon which looks like a piece of paper.

You can also try running the Apple Diagnostics to see if any hardware issues are detected.

There is a possibility that the internal drive failures are interfering even when booted to external media. Do these iMacs have the same problem when booting into Internet Recovery Mode using Command + Option + R or from a bootable macOS USB installer?