I used to use RAM for transcoding and thought I was getting the best possible performance that way BUT when the change came to Plex where it was claiming the temp folder didn't have enough room (it was seeing only whatever was reported as free space in RAM) with it showing up at my end as some titles just refused to play, I switched back to using the SSD (same as docker is installed on) and all those errors went away.
Now the followup to the "BUT"... when using RAM to transcode, my remote users had frequent issues with buffering and freezing. Even at relatively low settings (3 Mbps over a 30 Mbps connection). Once I switched back to using SSD, it all went away and they now can stream at higher settings with no issues.