I've done some digging. This problem has affected other phones.
Cause: CPU clock is likely throttled into a power saving mode and is taking its sweet time waking up.
Likely things that could exasperate the problem. Using live backgrounds, using custom CPU throttling software such as setcpu leaving your phone in a power saving performance setting.
Solutions: Flash to last V4 rom 3.2.28 (or wait until tomorrow's release of 3.3.15 v5), wipe cache and dalvik cache, remove any services that modify CPU speeds outside of MIUI standard, leave your phone in normal performance settings, remove any live backgrounds or high resolution images from start screen.
Honestly, I'd make a nandroid backup, then nuke my active system, data, cache, dalvik cache partitions and install 3.3.8 or wait for 3.3.15 and start over from scratch. It's honestly worth the effort to update to v5. That and a complete software refresh will either fix it, or tell you that you have a defective phone.
Sometimes I have the same problem with all roms. After receiving a phone call, I can not unlock the phone because touch screen doesn't work, although the power, volume, back buttons still work.
Don't know it's hardware or software issue
Thank you all. Problem has been probably solved. I set the volume buttons as a power button and that is why i couldn't unlock my phone sometimes. I think there is some "crash" between my unlock screen and the quick music settings.