People who complain about flagships like S4 and S5 lagging. Please don't blame everything on the phone, it depends also on your usage and what apps you have installed.
Yes, TouchWiz is a bit laggy with slightly aggressive RAM management but the phone is not to blame for this.
I know of many people who own the previous gen flagships and have apps like the following installed on them:
-Memory Booster
-Performance Booster
-App/Task Killers
-Battery Saver
-Clean Master
These types of apps are the main causes of lags, and those 1-2 seconds wait when opening apps. These apps basically kill all processes as soon as you leave them, free-ing up more RAM but slowing everything down since all processes have been killed and need to open-up and restart themselves. Constantly running applications do this, such as Facebook, Facebook Messenger, Snapchat, Viber and many more.
Battery savers do the same, however they try to stop the apps from starting after being shut, but since this isn't possible without root access, battery savers and performance boosters basically perform the same task, killing processes and trying to stop them from restarting.
Clean Master is something I have seen on all my friends' phones. It clears cache, kills apps, background processes and much more. This greatly affects the responsiveness of the device.
What I can recommend:
-Delete all the apps I mentioned in the list above.
-If a firmware update was installed on your phone OTA, I would ask you to perform a full hard reset including a cache wipe and then re-install signed apps from the Play Store.
-Dont kill apps all the time. Once or twice a week, restarting your phone is a good practice.
-If battery life matters to you a lot, then disable Auto-Sync entirely. However, keep in mind, this will require your data (contacts, gmail, phone data, app data) to be synced manually when there is a working internet connection.
-Reduce animations (in Developer settings) from their current setting to 0.5x if your device is fairly new (2013 onwards) or 0x if your device is old or you hate animations.
Lastly, I would like you to know that my secondary device is a Snapdragon 600 Galaxy S4, the LTE variant. It works perfectly on Stock 5.0.1, with a hard reset performed immediately after the update.
I have apps like Facebook, Messenger and WhatsApp on it, and a few more including Tapatalk, and the phone is not laggy at all.
In fact, it almost as quick as my S6.
Sent from my SM-G920F