Maybe i'm missing something but just adding the ScriptManager to a page (infact my MasterPage) seems to make the page take around 7-10 seconds longer to finish loading. This is with no Ajax Control Toolkit controls on the page.

 I've had a Google around and i've tried deleting my temporary ASP.Net files, turns the web.config debug setting to false.

 I made sure i built the toolkit in Release mode but since i'm not actually using any controls from it and i still get the slow loading i'm not sure what the problem is.

 (on the same Webserver pages loads instantly pretty much until i add the ScriptManager).

 Any ideas would be welcome.

 Thanks in advance

Try Firebug Network Monitoring & YSlow to precisely detect the bottleneck.

Read about Network Monitoring using the Net tab in the Firebug Firefox extension here - http://www.getfirebug.com/net.html

and suggestions on speeding up page load time  from YSlow Firefox add-on integrated with Firebug - http://developer.yahoo.com/yslow/

It won't affect the initial load, but make sure enableCaching and enableCompression are set to true in your web.config.

I'd second the suggestion of looking at the net requests in FireBug or Fiddler.  You're probably going to find that it's due to the ScriptResource and WebResource requests that ASP.NET AJAX uses to load its JavaScript, but if you inspect the load order you may find available optimizations. 

thanks for your answers. I've tried Yslow and it's telling me that i am loading:

 webresource.axd 22kb

scriptresource.axd 85kb

scriptresource.axd 30kb

I don't really want to make people with slow lines have to download that much. Is there any way of significantly reducing this? If not i guess i can't use Microsoft's AJAX extensions for what i'm doing.

If you have compression enabled, it's going to be less over the wire.  FireBug/YSlow show uncompressed numbers, not what was actually downloaded.

If you have caching enabled, the users will only download those files once per visit at most.  Often, they'll even be cached across visits by the same user.