I switched to ART right after I got the device. It seemed to work well for some time, but after a while, I would get slow response at times. Things like delay to get the screen to turn on, and other general lag at times. Probably due to some app issue with ART, so switched back. Since switching back, all lag issues have stopped.
It could be an app issue like you're assuming, or it could be one (or more) of the services you have running in the background. I've been using ART for over a week now, and everything is still just as fast and without lag as when I started using ART.
I tried it this week, found it very very slow surprisingly! I'm not sure if it's the sense software, but the phone was seriously laggy. Switched back after 5 or so days.
I doubt the Sense software has anything to do with it. If Sense was the problem everyone (who hasn't changed their launcher,etc) would encounter the slowness right away. I haven't changed from Sense, I'm still without lag, so its highly likely that your app(s) or service(s) are the cause of your lag.
For reference here is what is shown as running on my phone in App manager:
Settings, smartcard service, google play services, htc serrvice pack, speak, cloud (but nothing is set to sync for it), message+, my infozone, verizon support & protect, google search, verizon location agent, kid mode, and htc sense input. Therefore those can be ruled out as issues.