Even, the free vpns work with good enough speed and that's why I don't think there's much need to purchase a VPN subscription if it can be used for free and works at the same time.
If you are real gambler and gamble regularly, using free VPN should avoided because the IP given by free VPN might be used by other players as well.
Means that your account may connect to other account(s) and it can be something against the terms by the casino, once your account is suspected then high likely your account will be banned.
Paid VPN is not that expensive for real gamblers, so it should not be a big issue for gamblers who want to use VPN for their gambling activities.