By your question, there is 2 possibilities:
1. Your phone is lower end type.
2. You use slow internet or data plan.
How to fix:
1. If it is your phone problem, easiest is to get/buy a better performance phone.
2. if not, try close all other apps like fb, massanger, whatapps etc from running. remember to change setting to not re-start after you close it. min 1 GB ram free is much better
3. Clear at least 25-30% of total memory (free space) Phone memory. this can help slightly.
4. i had experience phone overheat after long period of usage (for lower end phone) which the performance, speed drop.
close all apps, network, gps, face to air con in car or any place cold. let it cold down at least 15 min.
5. if it is due to your internet problem, get a better internet provider, nothing else can do about this.
Hope it help.
Edit 1: sorry i miss out you said about your phone is iphone 6...
your phone may have too many unnecessary apps running in background.
