I have been experiencing this on my 735 since I installed Whatsapp a few months ago. My phone has been through a restore as well in that time and it does not fix it.
It is SO annoying and pretty bad. I sent feedback to them as well as bad review in the store so hopefully it gets some quick attention. Basically anytime you I use Whatsapp and then leave the app and then come back to it, there is a good 5-7 second delay where it just says "Resuming". The worst is when if I was using Whatapp and phone display timeouts while it was the focus app, when I go to use my phone again there is a big delay like 7-10 seconds of blank black screen. I am forced to just force close the app just to save the time to use the phone. Then when I go back to trying to open Whatsapp again many times it also just crashes on startup and have to click the icon again.
I have been using Whatsapp more as more of my circle have gotten it, so this is getting tiresome for me. Between BBM being removed from the store for good, now Whatsapp almost unusable and Gasbuddy stopped working reliably, I have sadly seriously thought of moving on from WinM10.