This is something I have been thinking about for a while and working on for a little bit:
It will be part of the forthcoming Smirk Wallet v0.3.0, which will also feature release of an open source backend so you can run your own backend for the truly privacy conscious.
The idea is the same as Monero LWS - instead of the server sending all outputs to the client and scanning happening client side, the client registers their hash rewind key with the server, and the server constantly monitors for them, so when they want to use their wallet it is synced and ready to go more or less instantly.