I would like to clear some misconceptions here. Rooting a phone does not make a phone faster. It just gives you a higher privilege of removing any or everything on your phone - like a administrator on a windows desktop. However rooting does have its benefits like you can remove system apps which one does not normally use freeing up some valuable space.
As per users asking for more free RAM, according to android philosophy, RAM free, is RAM wasted. So by default the kernel tries to keep all the apps in RAM once they are started. If they are already in RAM, showing them when required is faster, resulting in a better user experience. But, all phones do not have unlimited RAM. So next time when a user tries to open up a app, which is not in RAM and there is not sufficient RAM available, the kernel decides of closing the app which is already open in RAM but least used.When this happens, a lag may be observed. So do not worry if the free RAM is less. Its just the kernel trying to give you the best user experience
This leads to another common misconception, clearing apps by using task managers and battery savers. All this will do is the kernel will again start all the apps. This will result in higher battery drain since the processor will have to wake up for starting all the apps. Also the phone will experience lag till the time all necessary apps open again.