There's always massive parallel computation and new technologies such as vacuum transistors seem to come along to help Moore's law persist. Whether we'll ever have quantum computers with trillions of qubits, I don't know, but I wouldn't take bet against it.
If one's simulated persona is uploaded and effectively immortal, time in the sim can run at whatever rate is necessary to accommodate the required degree of realism. One's original body is long dead, of course, but if the integration of the physical consciousness with the simulated one persists for long enough, it's conceivable that it might be moved between the substrates and one would perceive one's existence as continuous.
Doesn't "quantum" imply that the uncertainty principle is involved in it? Maybe a computer that would make random mistakes in a completely unpredictable way will turn out to be impractical.