@lukeah: Try to avoid having too many apps running in the background, as this can potentially lead to performance issues. With this in mind, regularly tap the Recent Apps button on your Navigation Bar and hit Close All, or swipe upwards on any apps you're not using. Also, head to Settings > Battery and Device Care > Optimise Now, then tap Memory > Clean Now.
For battery issues, tap on Battery > Background Usage Limits > Put Unused Apps To Sleep > On, then use the Sleeping Apps and Deep Sleeping Apps options to further reduce the background activity of apps you don't use often. Tap the Back button to return to the Battery Settings > More Battery Settings > Adaptive Battery > On.
If the phone is still stuttering/lagging, head to Settings > Display > Motion Smoothness, and set this to Adaptive. Let me know if you notice an improvement.