For me it is like I told Aero, I play most Boss Fights on slow speed, so I can react better to the different Spells they are using.
When killing creeps or working on riddles, I play fast. I play on medium speed very rarely.
mfg TTH
PS: The whole question arises because I notices that in chapter 8 the Solo Fight of Phodom against Gaomdo does Bug, if you set the speed during the fight to slow. (Would be nice if some1 else could check, to verify thats the reason)
PSS:Yes usually the Game Speed should not effect the campaign, but all the Fights in TCO are complicated triggers. And if this triggers use data from the game input, the Speed Setting might make a difference and as I said I think thats the case with Goamdo.