Maximize app performance by increasing memory usage

Well contrary to the myths peddled by the snake oil salespeople who promote cleaner/booster/optimiser/battery saver apps, purging apps from memory does not save battery (it tends to do the opposite), so there's no conflict between what you ask for and that last objective .

However, unless you are rooted there is essentially nothing you can do. You could go into Developer Options (a menu you have to enable by tapping on the "build number" in the phone settings) and check that the "background process limit" hasn't been artificially restricted - but unless you have done it yourself using that menu it won't be, it will be set to the "standard limit". But that's basically all you can do.

If you are rooted, have a custom recovery (must be built for your phone) and someone has developed a custom kernel for your phone with a better RAM optimisation and which is compatible with the ROM you are using then you could install that. There may be a few kernel settings you can tweak with a suitable app (I had apps in the past that allowed you to adjust a few things, but I can no longer remember any names). But if you are not rooted or unable to root your phone, or there is no development for that model, then there's not a lot you can do.