Burning Speed Affecting backups??

[yop83]

Hi!
Is it true that burn speed (eg: faster than 4x) is making backups unplayable or messed up in some way?
WHY????

Here's my little story:
I bought a new burner (LG GCE-8400B - 40x/12x/40x) and tried to make a backup of a game from an ISO I had... The ISO worked correctly with Epsxe (PSX emulator for Windows), but the backup made with NERO at 40x didn't work in Epsxe (green screens flashing during MDECs) nor in the real PSX (never started up)...

Now, my Burner isn't capable of writing slower than 8x, so I passed along my ISO to a friend of mine who could write at 4x... His copy (made on a mac with toast) worked in the PSX, but MDECs skipped horribly... (He had made several copy in the past and depending on the game/luck/media used sometimes it skipped and sometimes not....)

So is the burning speed really a determining factor??
If so, why?
Shouldn't a faster CD-R drive just write faster and not make more errors or do anything different than a slower one??
Is it also a question of the media brand (I'm using Silver Sony CDs and 48x Mitsumi both 80 minutes) or quality??

Please, enlighten me on a way to burn correct PSX copy on a modern writer which does not write SLOWER than 8x (most of those sold today can't)...

Thanks!
Yop83