Greetings.
Welcome to the world of photographic scenery in Microsoft's Flight Simulator.   
FS9 has a landclass engine that does well at choosing from a relatively small number of different default scenery tiles when it comes to "painting" the ground. Add-ons like Megascenery (which I own) and Switzerland Professional (which I do not) , include thousands of different individual tiles that really tax FS9's ability to choose the correct tile and stick it in the right place. This is all really a bummer to me, too, as I was very much looking forward to Megascenery's release of the S.F. Bay Area, but as things stand, I would much rather have default scenery that loads up fine, than photographic scenery that won't. Kind of destroys the illusion. This is something I'd really like to see Microsoft fix in upcoming versions of their flight sim.
In the meantime, here are some tricks you can use to give FS9 a better chance at keeping up.

http://forums.avsim.net/dcboard.php?az=show_topic&forum=177&topic_id=356&mesg_id...

Hope this helps! - Matt D.

Well, poop.  ??? In that case, all I can suggest is contacting the makers to see if someone else had the same problem. - Matt D.

okok. i got it. they in fact WERE reloading BUT so  unreasonably(!!!) slow that even with a paused sim, it took the hi-res 15min+ (!!!) to show up on screen. [and that's with a defragmented hd and all the other hardware- & bios- tweaks that people talk about]

unfortunately, if you install switzerland pro, it will tweak your fs9.cfg to the bad, so all your 'good' settings go mayhem. additionally, the installer adds a bad entry for either TERRAIN_DEFAULT_RADIUS or TERRAIN_EXTENDED_RADIUS (can't remember) where it leaves the '=' sign out so that the line says TERRAIN_DEFAULT_RADIUS9 or so. great job. really. luckily, fs corrects that with its own values back to normal with the next run. but again, you have to check back on that manually. to have your preferred values set.

i followed madmatt's link by the word, checking performance after every change. two key issues that solved it for me: the extended textures entry gets set to active by ch-pro, which you want to turn off and i had the framerate set to 25 instead of unlimited. that's basically all i had to change. all the rest was already with the optimum.

additionally i got word from flylogic and they told me that the radeon9700 was simply too slow and that they suggest a 9800ultra for ideal performance. but even then, dont expect textures to load fast enough when you use jets.

at this point i'm happy with the situation. fast turns might get the texture system to 'lag' on me, but in general it's performing well for any slow aircrafts such as the cessna.

i just want to stress once more that one false setting (even if set by the installer) might wreck everything! so if you have this problem, go back to the books and start your config from zero. one of them must be wrong.

greets.