Users should be able to read their existing history rapidly when logging in on a new device.

[BillCarsonFr]

Large key backups take forever to restore (or fail entirely); should restore incrementally instead

### Web
- [ ] https://github.com/vector-im/element-web/issues/19778
- [ ] https://github.com/vector-im/element-web/issues/23359
- [ ] https://github.com/vector-im/element-meta/issues/2176

### Rust
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/1267

IOS: Similar issue

Android: Better but still some issues, incremental backup calls done after each syncs so can take some time if not a lot to sync. And it's not working that well for large gappy syncs, in this case the crypto service is catching up on a lot of to_devices that will trigger upload of the newly received key and looks like it's blocking the backup from incrementally downloading keys

How to test:

• Login an existing account with a big key backup.
• Open e2ee rooms, history should decrypt quickly (seconds)

[richvdh]

requires a fix to https://github.com/element-hq/element-web/issues/27009 ?