What's the difference between the xtrem42a.bin and xtrem42b.bin hacked firmwares? I know it has something to do with read speed of the backup, but I can't find any explanation in any of the many tutorials I have read. Somebody, please educate me.

It's right there in the readme.txt...

4.2A: default read speed is "Fast Read mode 12x" for backups
4.2B: default read speed is "Quiet/Slow 2-4x" for backups.

Original disc read speed is unaffected.

So it all depends on if you want "fast" or "quiet" DVD-ROM access by default.

Is Original Disk Read speed always 12x?

Is 4.2a the more popular choice then?

I used 42a and it read fine. When I reduced the speed to slow it wouldn't load the game. Admittedly it's louder than the slower speed.

All originals run at normal speed.

What is "normal speed"?

Normal speed/Factory speed/12x Sorry for the confusion