The UI is on it's own thread and shouldn't be getting blocked, but there are probably some statements in there where it thinks it's making a quick function call but due to a bug that function is not returning.

If I didn't implement async in the main code, then everything would be on the same thread, using a single CPU core, and all activity would be linearized... I'd have to wait for one relay to respond before I could ask the next relay. So async is vital in general. If just the signer wasn't async for its NIP-46 part, all operations like sign, encrypt and decrypt would block until the nip-46 relay replied, and that would slow things way down, but the UI wouldn't freeze.