What are your computer specs? I highly doubt this is the issue as you said you ran the SF version fine. But just curious as I just noticed you said that it isn't too bad when you are offline. When you fly around the scenery when you are NOT on VATSIM, what kind of frame rates do you get? Is it still a slide show?
Before doing what is below, let me know the answers to the questions above^^.
i did uncheck it in the FS9 scenery , do you think that would make a big difference. i didn't wanna remove it cause if i want to go back to it for some reason.
Probably just delete the area and move the scenery folder out of Flight Sim folder. Or, I suggest just deleting it. If you bought it from SimMarket, you can always redownload the product and they will still have your key under 'Your Account'. Or you can put it on a CD or Flash Drive or something and delete it all off your computer.
However, I still think the root cause of your probably is possibly a duplicate AFCAD conflict. So follow my instructions below.
Download this program here: http://flightsim.com/kdl.php?fid=71966
(you do need to create a free account if you don't have one - well worth it as you'll then have access to TONS of great FS addons and sceneries and things)
Install and start up the program. Then click the little 'Open' folder icon button. Let it collect all the airports (it may take a few minutes). Enter "KLAS" in the Airport ID field and click search. If you see two AFCADs labelled 'Mod', then you might have a problem. Double-click on either Mod one and then right click--> 'Properties'. Find out which folder it is in. If it is in the FSDreamTeam folder, you're all set. If it isn't, try moving it out of where ever it is onto the Desktop.