Hi all - a while back my quick selection tool became very slow and the lag was so bad that i stopped using it.  I got a solution today via Adobe Forum :-)
 
	
		Go to the Preferences of Photoshop, then Performance Tab.
	 
	
		In there , please click on " Big and Flat " , restart Photoshop and then check the Quick Selection Tool.
	 
	
		This worked for me - I hope it helps.