What are some steps to diagnose what is slowing down my LG G2 D800? It just took me 40 seconds to switch from Facebook to my SMS.  (I got a message while I was in Facebook App, and I clicked on the SMS pop-up and it took good 40 seconds to make the switch)

While thankfully not everything takes this long, I am curious to try to diagnoze what could be the issue.  I have no idea where to begin.    Most other things take anywhere from good 2-5 seconds, like app switches, email send-offs, etc.

Can I have some troubleshooting steps I can use to make some steps towards speeding up my phone?

Factory Reset does help, but I have done 3 of those in the last 1.5-2 years and I would rather do something else now.  Maybe figure out what the issue is rather than just postponing it again via Factory Reset.
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!