Galaxy S3 high RAM consumption even on idle I have a Galaxy S3 International Version, without any modification whatsoever-- no rooting, no nothing. It is as good as a stock phone with a few Play Store apps.
However, of late, I have begun experiencing problems   

I do not have background processes (of my own) running and even then all I have left free in the RAM is usually only 300MB of the 800MB that is there. Task killers cannot free more than that.  

I experience lag when opening settings. What I mean is this: Say I click on the Battery option under Settings. There is a visible lag between the time I press the option and when the battery information is actually displayed on the screen.  

Android Version: 4.1.2
Build Number: JZO54K.I9300XXEMF6
First a few things on the symptoms:


On Unix/Linux/Android, "free RAM" is considered a "wasted resource". If RAM is not occupied by running processes themselves, the system uses it to buffer and cache other things, like the file system. This should speed things up, as usually RAM allows faster access.
For more details, please check out ram tag-wiki.
"task killers" are discussed controversely. While most (if not all) people agree they are fine to stop "rogue processes", it's certainly not their job to "free up RAM" or even "save battery". In these terms, they are rather contra-productive, as most killed processes simply restart themselves (which costs more battery than if they'd kept running, and fills up the "freed RAM" again as well).
For details on this, closer information can be found in the answers on e.g. Do I really need to install a task manager?, Do task killers actually work? and Is it advisable to run a task killer app on Android.


And finally, to the issue itself (experiencing lags). As you describe it as a problem "growing with the time" (i.e. it was not there in the beginning, but slowly became more and more visible), you will probably find some help in the answers to My device is getting slow, apps start misbehaving/crashing. What can I do?