What speed should be chosen to get the most accurate results? I've read that low read speed helps get the most accurate results possible. Is it true?
Not really. Lower reading speeds sometimes, though rarely, assist with copying cds using a subcode based copy protection (e.g. securom, securom 2 & psx libcrypt, all of which are thoroughly out of date). It doesn't assist at all with copy protections based on unreadable sectors (e.g. safedisc [versions 1-4], laserlock, ProRing [aka RingProtech], all of which are also out of date.
Is the CD image a perfect clone if I encountered no read failures using max read speed?
If the original cd was copy protected with an unreadable sector based protection [see above for examples], definitely not as you are supposed to get read errors when making the image.
If the cd was protected with a sub-code based protection, maybe yes and maybe no. It depends upon whether or not the sub-codes were read correctly.
If the cd was unprotected, almost certainly yes.
Generally, you might wish to have a look at the various guides in this sub-forum.