From what I've seen, foss web clients are severely lacking. Personally I use nextcloud. SOGo looks interesting, would probably be more popular if it was easier to run (mailcow has a docker image, although you'd need to dissect what is need to just run SOGo).
Can you define "slow"? I've seen many a person here complain about Nextcloud's performance. For context my instance page loads in around 2.5s. I don't use Talk, Mail or the dashboard.
Proxmox LXC 2core (4700u), 4gb ram, ssd, linuxserver's nextcloud docker image, mariadb, keydb.
It also runs OnlyOffice, Drawio, excalidraw and 3 or 4 smaller services