https://www.chess.com/blog/News/how-to-adjust-your-live-chess-connection
That may help and the following, while about disconnects, has information that van assist with slowness.
https://support.chess.com/customer/en/portal/articles/1444918-my-internet-connection-is-fine---why-am-i-getting-disconnects-
With millions of Live games a day, many of them bullet and blitz, many people play OK.
Premium???
With Chess.com's slow and erratic connection speeds blitz and lightning games are impossible. No way I would pay for this.