There are potions of the same name that also won't stack together
if you created them via alchemy, and if your level was higher than 1 => these are stronger, so they don't stack.
About arrows => some are a bit different, e.g.: Poison arrows and "Poison arrows of the incredible hermit" (made this up). Please, check if your arrows really have the exact same name.
About lag => sorry, no info. I play with a 2800 Athlon XP, and over 1 Gig RAM, no lags, though I have 2 graphics programs, WORD, the internet and countless files open in my task bar.
