[Mee-n-Mac]

I'd have to review some reports to see if this makes sense but ... I thought the GPS receiver remembers where it was when it was last on. Based upon that info and a database of the satellite orbits (ephemeris data) it tries to acquire those satellites which that it calculates can be "seen" and not those below the horizon. Once acquired, the satellite's true position is received from the transmission. So I'm guessing that (due to drag and other things) the stored ephemeris data becomes out whack with the true oribits and results in slower acquisition. Same thing happens if you move the GPS receiver when it's turned off, it takes longer to acquire from it's new position. Loading the receiver with newer, updated satellite orbit data restores the quick times. Brandy new units already have a "good" database and so don't suffer (right now) from this problem. When I get a chance I'll see if this is more than just a guess.

[Edit : Did some research and I'm no longer sure the above is plausible. See follow-up post]