Did you try to clean your disc? If that doesn't work, try to get any dust out of the PS3's disc tray. You might want to test a different game, too, just to check out whether it's the disc or the PS3.
When my internet connection is playing up GT5 takes AGES to load, try signing out of PSN and see if it helps at all. Probably way off the mark but it's worth a try, maybe.
I think it could either be your internet connection, like what the above poster said, or maybe your hard drive. Mine has been giving long loading times as well, even when signed out. So, I decided to change the hard drive. But don't take my word for it, I'm not an expert.
When my internet connection is playing up GT5 takes AGES to load, try signing out of PSN and see if it helps at all. Probably way off the mark but it's worth a try, maybe.
I have this also, when I have issues loading GT5 I check my net speed then I turn the PS3 off, turn off modem and wifi router wait 15 mins or less restart computer while modem and router are rebooting and then check net speed and then start GT5 I notice that it loads instantly with a revived (strong) interent connection. No idea why but it does
It would most likely be internet problems, since there are times where it would take forever to load up and other days it would be done within seconds. Only thing I did was turn something off to loosen up the bandwidth.