I recently migrated from Element to Element X with first the proxy and now simplified sliding sync, and it feels way faster. Imho Element X is still very alpha software, so I wouldnt recommend it to the general population just yet (and I still occasionally have to open the old Element), but the speed is really noticeable on even a very small instance.

Sliding sync was recently implemented in Synapse itself, so as long as your Synapse is up to date there shouldnt be any more setup on the matrix side. Try checking the Synapse logs for any issues and/or the cloudflare tunnel configuration (I have no idea about what it does to traffic).