Some models are too high poly for Brawl to handle. There's even slight lag when there's 4 Ice Climbers in one match.
Your only ways to reduce the poly count are to Optimize the model, which might ruin the model if not done right or use the Tristripper feature with the newest BrawlBox, this is the easiest and most recommended way to reduce the poly count if it's too high. Though for some models, using the Tristripper alone won't do any good.
The max amount of (polygon) faces Brawl can really handle is around 16,000 - 18,000 faces with minimal or no lag.
The model isn't too high poly for it that I downloaded. I downloaded the Johnny Bravo thing for ganondorf and it didn't cause lag before and the .pac is only 0.3MB.
The size of the FitChar0X.pac/FitChar0X.pcs does not matter.