Apps close when screen is locked and restart during multi tasking

Hi all. So a few annoyances with my iPhone 6 Plus 64gb (Currently using 12gb of storage). I would have thought that the 6 Plus would be able to handle multi tasking at a much higher level than the previous iPhones, but to my dismay, it still *****!

Firstly, let's say I'm using an app or maybe a game and I decide I need to go make a quick drink or use the bathroom or something, I lock the screen with the app open, go about my business for a minute or two and when I come back, the app has closed and I'm at the home screen again. I've actually replicated this many times and managed to lock my screen accidentally once and unlocked it straight away and the game I was halfway through had closed! That's in 1 second!

it does appear that this happens more often with the memory heavy apps and games But happens even if that's the only app open and all background apps have been quit.

Secondly, multi-friggin-tasking! Does that mean I should be able to switch between apps without them constantly rebooting in the background?? A prime example would be transferring information from an email to a spreadsheet in Numbers and maybe you get a message through with a safari link you need to quickly check out. Could be only seconds in between switching in and out and yet when I go back to email or numbers again, they're right back at the beginning! Again, this can happen if there are many or only a few apps running in the background.

So your thoughts ladies and gentlemen? These are just a couple of the annoyances I've found so far, but I'll post the others separately.

Cheers for reading.

Dave