I don't have a workaround.
I think nostr:npub1sn0wdenkukak0d9dfczzeacvhkrgz92ak56egt7vdgzn8pv2wfqqhrjdv9 has experienced similar issues with his client (iris/daisy/?). A relay doesn't need explicit support for tor, but it mustn't block tor exit nodes. I suppose some relays are run at ISPs which block tor exit nodes by policy. It would be good to determine which relays are like that.
Tor will be slower because the service of onion routing isn't free, and network traffic is the slowest part of modern computing.