Input lag for some users is fairly common with script laden database intensive sites. Especially common on forums that run third party ad scripts.
Google reports that vBulletin has much the same gripe from users. There are a myriad of reasons for this annoying behavior. Gee, where to start? Anti-Virus/Malware apps or "cloaking" software or browser add-ons for instance can rob CPU headroom and cause issues like this.
I don't know your particular setup, just tossing that out for something to consider. And yes, it can happen to you here but nowhere else because, well...everyone's computer and Internet is a complicated stew of unique scripts, servers, software, and browser add-ons.
FWIW, personally I'm having no issues with my daily drivers Chrome or Firefox. I fired up IE and while it was a slower to load, (no surprise

) once she finish crunching the scripts, I have no problem with input lag on AK. -->Latest version of IE on Windows 7.
Just for shits and in the sole interest of trying to lift you/others out of a ditch....fire up yer rig in
Safe Mode with Networking. Open IE and take AK for a test drive. How'd it go?