It's a combination of both, actually. MicroB is partition hobbled by slow hardware combined with very demanding websites and web technologies, but it's also a function of the very old version of Firefox 3 that MicroB is based on being utterly inefficient at nearly everything.

But that, of course, has very little to do with the competence and capabilities of the managers, architects and engineers involved in the product. Which is why saying "MicroB sucks" with no further qualifications or information is useless and harmful.