Only 3 weeks doesn't bode well for the types of things that usually cause issues like this... but here goes.
Most often lag is caused by system resources running low. There are a lot of ways this can happen, however honestly they usually take longer than 3 weeks to appear however each user is different so this can vary a lot. The simplest things to do for system resources are;
Delete SMS and MMS messages from your device. Managing the number of messages, more MMS than SMS but every byte counts, can contribute to a slower device. Your app will try to make those messages easily available whenever called on and therefore may hold too many in your RAM. This is really dependent on your device and your app, and how well they play together, however reduction can show surprising results. There are several backup apps if there is anything you are worried about loosing.
Clear your caches. There are so many ways to do this but usually the simplest way is to restart the device. Often the caches are held too long because the app never gets the signal to dump them. Then it fills it quota for cache content and tries to overwrite it again and again. Powering off your device or restarting it using the menu option offered by the power button is the easiest way to clear most caches. You can however take a more hands on approach to this and manually clear them by going into your App Manager and selecting apps to clear caches in. The big 3 are Facebook, your email app, and your SMS app, so if you are going to manually do this make sure you hit those. There are also a ton of apps that will do this, a few words of advice though; Don't pick an app that will do this automatically based on time or anything like that. While they make it sound easy peasy apps like this can cost you battery power or memory power, maybe not but could. Simple is better and picking one that you have to press a button on is not such a bad thing.
Along the lines of the cache clearing also look at any stored emails on your device. Some apps store sent emails on the device by default. Again eating up these resources can slow things down. Try to change this setting and or deleting sent emails from your device.
This one is often over looked but what is installed on your device? Too many times we take for granted what we install on our devices not always realizing that it does matter. In your App Manager what is running? Anything that you didn't start or haven't used in a long time that seems out of place? Again the simple solution to this is restarting the device. It will often kill unwanted apps, some of which may return, but it is the simplest way to knock off the ones that aren't needed. Swiping them away from your multi tasking screen does just that, swipes them away, but doesn't shut them down always. Here is where I advise you to stay away from Anti apps. Anti virus, Memory Boosters, and Battery Savers. These are all apps of the past and have you ever heard of an Android virus? These apps can have the kind of adverse effects that you are experiencing. Memory apps can shutdown or hinder processes that are vital to smooth running, the same with Battery apps. And don't even get me started on Anti Virus apps. Huge scare tactics. Just saying. You're more likely to have a rouge app that slipped a permission by you so check your recently installed apps. Open your app drawer, in the upper left hand corner choose to sort by most recent and look at the last app you installed. Anything there that you could live without? Anything there you didn't read any of the 1 and 2 star reviews on. Always check the 1 and 2 star reviews. The good reviews can be faked, no one is going to fake a low rating review. Look for key words like, Slowed, Bogged, Lag, Pop-ups, Always Running... these are strong indicators that this may be one to pass on. Try uninstalling anything that looks out of place or that you really don't use and see if things clear up. Be sure to restart your device after an uninstall to clear out any cache it may have left.
Finally restart your device every few days. We are so glued to these things that we rarely take the time to power them down unless they die in our hands. It's simple takes just a few moments and does a world of good.
GL
I mean I should never, ever have to reboot , right?..... I reboot every morning and have no lag or other issues. Takes 30 sec of my time. A possibly simple fix, try that and see if it helps.
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