Not sure how well this works out but I thought it seemed like it might work ok.
As Ellyos explained somewhere, the character level compared to guild level has a lot to do with how much experience the guild gets from its %.
So my thought is to raise a character to the guild level for maximum efficiency, and then put him to 90%. He would be sort of a guild exp slave, who many people have access to, and they can grab him on exp runs when ~21 levels is pretty easy to accomadate.
This way, you can contribute large exp to guild without slowing down your own levelling. Dont know how efficient it would be in practice, but might be worth a try