I have noticed more lag time with my iPhone in general lately (both typing online and in texts and general use) - seems to have gotten worse since I updated to IOS5 - but I haven't noticed it specifically worse here. Simple Machines could also be the explanation as CM suggested. I believe I can download Tapatalk capabilities to our forum, but I wasn't sure if people really used that app. I've not used it and haven't felt the need to buy it, but if many of our users do have it and feel it would make their phone browsing experience better, I can see what I can do about installing it on our site.
One other suggestion - have you tried using WAP2? Is it still laggy? I don't use it because I prefer the full site layout, but it might be a temporary solution for you. At the bottom of the page, in the "credits" section you can click the WAP2 to try it.