It’s quite normal to experience network latency when using VPNs, especially when it’s a free one. This can make your gambling experience frustrating.
If you don’t have any valid reason to use them, then just don’t. It’s not worth it.
When you use a Free VPN, it contains risk of many things. Firstly as you said, it's network latency that is inconvenient and uncomfortable but it's only about your individual experience. Another disadvantage, it can affect your playing experience and maybe your bet result too. Last one is severe, with free VPN, you can have more possible issues with an online gambling platform.
If you use VPN, for your web surfing, a free one can work well, but if you use VPN to access platforms where you store your money and play with your money, it's very dangerous.