Phones use Solid State Drives (SSDs) for storage instead of traditional Hard Drives (HDDs). This is because carrying and jostling HDDs can cause hardware problems.
Solid State Drives, however, get slow as you fill them up. So when you start to fill up your phone, its storage gets slower. This means that it takes longer to read the code from storage into RAM to execute.
It should also be noted that the description on Autorun Manager says that some apps will just attempt to restart themselves after it kills them. This constant restarting-killing behavior could possibly be even harder on system resources that just letting the background app run, so be careful which apps you kill.
Some background tasks help apps start-up faster too. I can start up a heck of a lot faster if I've already updated my app while you weren't using it. Instead of waiting for my app to get all of the data it needs when you open it, it grabs it in the background and can just display it when you open the app.
Also, if your device has very little RAM, then it can only hold a couple of apps in there at a time. If the apps you're opening have been erased from RAM, you phone has to erase another app from RAM and use that space for the new app.
Basically, there are a ton of reasons why your phone could be acting slow, and it's probably a combination of them that's causing your phone to act so poorly.
