Its a known issue. Fix is either to download a lockscreen replacement (Lock 2.0, WidgetLocker, etc.) or you can root and install a custom rom with a different lockscreen (Apex for example) Its really sad the moto even with the 2.2 update can't make a simple lock screen work correctly.
It is purley asthetics in my opinion. Having said that, a dead pixel is the same thing. It doesn't effect the functionality of the phone, but its always there staring you in the face. I lock my phone every time I put it down, so I'm always having to unlock it to use it. and the lag bugged the hell out of me. Luckily I'm running apex, oc'd to 1.38 and the rEVOlution theme so the problem is non existent to me now!
I agree it's laggy, it doesn't really affect my day to day use but it does make the phone feel... slow when it's not.
I'm trying out other lockscreens now. I wanted something clean, simple and smooth.
I'm trying Lock 2.0 Limited Demo. It's clean, and very responsive. I like that I can set it to use my desktop wallpaper and that it switches to a dark wallpaper when I plug it into the charger (doesn't light up the whole room at night quite as much). BUT... hitting the home screen button dismisses the lock. So if I accidentally hit the home button while the phone is in my pocket, I may end up butt dialing or butt surfing or... you get the idea.
Any other recommendations on clean, responsive slide to unlock lock screen that actually locks the device?
Widget Locker is the only lock screen replacement I've been happy with. It's well worth the small charge. It's extremely smooth, hasn't ever given me a problem, and even has the cool rotary lock screen as an option. I just wish it could replace the laggy slide to answer screen.