My question is basically this; will a game play faster if it's accessed from an iso or played from the CD?
I expect it makes negligible difference but I would say ISO if there is any difference.
Games that use to read from the disk a lot will give you speed boost but it's better to use ISO rather than CD on any game
ISO over CD will give you a huge speedup in games that read a lot. But any game reads so overall you will feel the difference in almost any game. Try it yourself and see what fits you best.