From about two or three weeks ago, I found that whenever I start Firefox and connect to VPN, it is fast for the first 5 minutes then becomes terribly slow beyond, and it happens very constantly. There is not the same problem on Chrome. I've contacted with the support service but no suggestions help (request #4257080). I believe it is the problem of the Firefox extension itself.

(update 2023/1/30)
It seems the problem has been fixed.

(update 2023/2/1)
Nah... problem's back, disappointed. I'm gonna use another VPN.