Video starts much slower than in original Firefox

[xisnamh]

Hi, I watch this online channel a lot https://www.mitele.es/directo/fdf/
and when I press play, it takes many seconds to start compared to the original Firefox.
Tested with:
Freshly installed Mercury + ubo extension activated (even the text of starting advertising appears but the advertising never appears)
Freshly installed Firefox + ubo extension activated

Could someone see if the same thing happens with the same steps?? It's something curious that I can't understand, I've looked at the configuration of Mercury and Firefox and with all the options the same as Firefox the same thing continues to happen... thanks!!!!