How to cut aluminum plate?

Ive cut up 1/8" with circular saw. Makes a LOT of noise. If the sawsall is slow , you probably have too fine a blade. 10-14 TPI should do OK. Lube with wax or oil can help keep teeth from clogging. What shape are you cutting? If doing a 'U' to slide over bolt drill the hole and then just cut the straights into it. Spade bits and hole saws work good in Al also if you need round cuts.