I cannot watch videos using wifi

Whenever I want to watch a video using wifi on my phone, I cannot. The video typically will not load or it is so slow it isn't worth it. I have to turn off the wireless and try again. This is very frustrating because I don't want to have to turn off wireless and I definitely don't want to use my data but short of getting a new phone, I'm stumped. Any thoughts on what I can do to fix this? Thank you.
