Problems with speed through wifi

I been with talktalk since 28th June this year, when my internet first went live I had no problems at all. But for the last week or so when on my phone connected via wifi I've been having problems on apps like Facebook. It seems to take an age to load pages and videos on there constantly buffer, this is not just on my phone the same thing happens on my partner and daughters phones as well.
Also tried changing the wifi channel but made no difference.

Thanks