I use retropie on a 4b and N64 sometimes lags, so I'm surprised you can play a GameCube game on the 5. However if you are new, I agree you should not overclock your 5.
RetroPie
RetroPie allows you to turn your Raspberry Pi or PC into a retro-gaming machine. This community focuses around the projects that come from this as well as providing tips/tricks and help in getting it setup.
##Current Version - 4.4## ####Download & Changelog####
####Faq####
#Useful links#
#Rules#
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Don't post links or ask for links to ROM or BIOS files. Also, don't ask for names of sites containing ROM and BIOS files. We know everyone has them, but let's keep these out of this sub.
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No thread crapping. People put in lots of time and effort into some of these projects and we don't want to discourage anyone posting.
There was a lot of work to get it up and running since it's not included with retropie but it seems to play fine aside from the few choppy bits! Dreamcast worked perfectly, I haven't had any problems there at all