Marshall Major II bluetooth latency

I've been having problems using my bluetooth marshall headphones with my iPhone 6s (16gb), especially in the youtube app. The sound delay between video and audio is really long and very annoying. This is the first time i've tried bluetooth with these headphones, which is the reason why I haven't noticed it earlier. I got the headphones as a christmas gift last year, and haven't noticed any other problems with them except that I can hear a vauge beeping in them when listening to music wireless. I tried conncecting the headphones to my computer instead and the sound workes perfectly, but for some reason it's slow on my phone. I can cleary hear that the audio starts later than the video, and can be heard after the video is finished because of the delay. I don't know if YouTube and Instragram has problems with their apps, becuase the sound works fine with Netflix. What can I do? I've tried reconnecting the headphones, rebooting my iPhone and updating the operating system to 11.2.5 but nothing seems to work.