How many browses you have on your iPad should make no difference. Only one can run at a time. Safari is the fastest (with a caveat) browser on the iPad. This is because it is currently the only browser that can take advantage of Apples new improved javascript engine. That might change soon, but no telling.
The caveat is Opera, as you've noticed. Opera works through a server that optimizes (compresses) content before it sends it to the iPad. For certain types of sites this can make it faster than Safari. It may also reduce the viewing quality of the site, but you pay a price for everything.