What are some steps to diagnose what is slowing down my LG G2 D800?
1) Try to free internal memory! Delete apps that you don't use, transfer photos to your pc, etc!

2) Disable unwanted apps! If you want, you can root your phone to remove bloatware, too! That helps a lot!

3) Use (one time per 15-20 days) a clean up app like cleanmaster, ccleaner etc! Delete it then, because daily usage it's not good for smartphone!

4) Clear cached date from apps that you use!

5) If all else fails, factory reset!