@patrick Basically, MJ only offers you correct DNS to access blocked sites. It shouldn't made your internet speed became slow.
Please try to connect like this : Telecom's modem -> MJ (maybe you need to change PPPoE mode) . If it works fine , I think you have to check your original router. your original router maybe set some firewall or rules to reduce MJ function.
Let me sync with you, you can send me a message of your destination website, I could check speed from my side. (Yes, of course, we do not exclude the possibility of broken equipment.)