Wondering which browser is fastest? These guys ran browser speed tests across an impressive array of operating systems. The hardware used is mildly obsolete by today's standards-- an 800mhz P3 with 256mb of RAM-- but there's no reason to think the benchmark results wouldn't scale to faster machines:
Many google apps have easy client side fixes using userjs etc. It would seem that google does not even try to get Opera compatability, and that would also explain why it often has none.