The alloys are very simple characters who are going to be very non-taxing on the Wii. Battlefield (the only course they appear on) is also a very simple and probably coding wise a very optimized level. The stage does matter a lot; go check at what a 4 Ice Climbers free for all on Big Blue was like in melee.
I'm not sure what the more taxing levels are in brawl because offline the game never lags (and if you do ridiculous stuff with special brawl and items, stage doesn't seem to matter). If we use online as a judge, Lylat Cruise is probably the "laggiest" stage in brawl, but it's not clear that's because it uses a lot of system resources (it might just be the nightmare of having to avoid desynchs with that continual random tilting). Still though, if you used seven different characters with full movesets, used the "hard" characters like Pokemon Trainer, Ice Climbers, and Olimar, and played on a level that was already more taxing on the game, I bet it would lag offline even with no items in standard brawl.
Also, I don't think stages other than Batttlefield have enough spawn locations. That sounds trivial, but try coming up with enough to make Final Destination work with more than 4 people.
