My dialler app is severely lagging I have a HTC Sensation running Android 4.0.3 (HTC Sense 3.6, 3.33.401.53).  Of late I've been having increasing problems with global lag, however this is really most noticeable on the dialler/phone app itself.  By more noticeable, I mean several times worse compared to anything else.  

I can open the contacts menu in the dialler without difficulty and filter by name to get the number I want up, however I get no response when I click the name (aside from a green flash of it to indicate I've pressed sucessfully).  There's then a pause of I would say up to a minute in the worst cases before it actually dials.  Usually I've pressed it multiple times in the hope of it working, whereupon it calls the person multiple times and I end up with several calls to the same person 'on hold'.  

Is there any known cause for this or even better any likely way to fix it?  I could do a factory reset if this doesn't get resolved as it makes the phone close to unusable.  

The effect isn't really on answering calls, though there is a lag on that too (but not to anywhere near the degree).
Having had no better suggestions I did try a factory reset and it seems to have resolved the problem entirely, indeed the phone 'feels' new.  Whilst it is annoying, it's been over a year since having the phone and it only took about an hour to factory reset and restore important files, so it's not the worst solution out there.