Till afternoon everything was fine but when I started my laptop in evening, I see that for a application to launch it is taking 30-40 seconds 
Initially it used to take atmost 5 seconds. What can be the reason for this and how do I rectify it ?
Impossible to say without a lot more information but some possibilities:
System wide virus scan was in progress
Swap file was resizing due to memory overuse
Processor has overheated and automatically underclocked
Virus or other malware infection
Windows update installation in background
As EastCoast mentioned overheating can cause that, and it’s surprisingly common if you’ve never thought to clean your fans. I spent an hour trying to figure out why my mother’s laptop was crawling slow before I realized one of the cooling fans had stopped spinning because it was full of dirt.
your laptop may be heating more than necessary
this can be due to fan failure
or your ram is getting old and has worn torn out
Right, been said twice now
RAM doesn’t get old and wear out.
Turn off laptop
Remove battery
press power button for 30 sec
Replace battery
Turn on laptop
Notice any difference? If not, reformat.
Re-install your operating system will definitely help you!