I connected my Haylou GT2 bluetooth earbuds to my Windows 10 Desktop. However there is audio lag when I watch Youtube. People are talking and their mouth movements don't match with the audio, it might be off by 100-500 milliseconds.
Bluetooth headphones always have additional latency because of encoding, transmission, buffering and decoding of the audio stream. The latencies I have seen have varied from 30 ms to nearly a second. Some video players are able to correct for latency, but not all. An additional problem is that while Bluetooth headphones are supposed to report their latency to the computer or phone to allow latency correction, some headphones either report wrong number or nothing at all. This problem is most often found in cheap headphones like Xiaomi, but some name brands do it too. For these, synchronizing audio and video is impossible without separate calibration.
Edit: According to OP's self-answer, disabling audio "enhancement" fixed the problem, and "enhanced" audio syncs correctly when played from built in speaker. The most probable explanation is that both the headset and "enhancement" have correct latency compensation, but when "enhancement" is used, the latency from Bluetooth is not used in synchronization. This is a bug in the audio driver.
You may have Audio Enhancements enabled. This causes a delay in the sound (in addition to the delay already present from Bluetooth headphones).
To disable this feature:
- Right Click the speaker icon next to your clock calendar at the bottom right of your desktop
- Select Troubleshoot sound problems
- Select your audio device, it should be listed as Haylou-GT2_R and includes (Current Default Device) in the description, click next
- Select Yes, Open Audio Enhancements
- In the Enhancements tab, checkmark Disable all enhancements
- Select OK
Your sound will cut out for 1 second and come back without lag
If the lag is consistent, it's likely that there's just a degree of latency. Some earphones will be a little bit out (like my own - on every device I own they seem to be about 50ms out). If it's not, try @zwei7's method of disabling enhancements. I don't have access to your model of earphones so I can't test this out, unfortunately.