Hi jai not a guru but muddle through with the help of this website like most of us. As you say this problem would be easier to understand if the cpu was running at a high percentage. You could try running 'Prism Hud' a freebie from cnet.com which shows ontop layer of screen what is occurring on your computer at anytime. Download and run it. This would give you an understanding of what is running. A good diagnostic starting point. It indicates current use of cpu, memory, hard disk volume, faults etc. It also indicates what program is loading up and shows if it sticks on a program, then that is a possible culprit for the problem. I wouldn't be surprised if it is firewall/anti virus related in which case an uninstall/reinstall may be required. Before you start the above just try running Ccleaner in safemode with networking and restart normally.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been