Thanks for replying. For what I've read, it seems it has something to do with this (it's from another thread here at adobe forums):
Photoshop CC 2014 changes stylus use to use Microsoft system API's, with these API's you need to have Windows Ink enabled. These API's give better stroke results and improve the out of the box experience for users using Windows tablet/convertible devices. If you want to go back to the old WinTab implementation of previous versions do the following:
Create a file that contains instructions to revert to the WinTab functionality.
Create a text file in a text editor such as Notepad.
Type in the following lines:
# Use WinTab
UseSystemStylus 0
Save the file as a plain text file named PSUserConfig.txt, and save the file into the Photoshop settings folder: C:\Users\[User Name]\AppData\Roaming\Adobe\Adobe Photoshop CC 2014\Adobe Photoshop CC 2014 Settings\
It's too complicated to explain all the possibilities, let's just say that for me, I have to choose between having lag, but no pressure sensitivity in mouse mode (using the pen in relative positioning), or the opposite. But it seems I can use pen mode and have pressure sensitivity without lag when switching to the physical mouse.
Anyway, I'm really tired of all this experimentations and unexpected behaviors. I'll try working in pen mode and cross my fingers.
Using CS5 or even CS6 without the updates is a better choice, no doubt. All the new bells and whistles do not make up for this huge and extremely annoying problem.
Anyway... thanks for replying back and best of luck for you in all your projects!