Sync times have regressed significantly since Ignition launch

### Steps to reproduce

1. Foreground the app in a big account
2. Stare at the sync spinner for 4+ seconds
3. Feel very sad that previously it was syncing in ~1s.

Kegan has dug into a rageshake, and is seeing an inexplicable 3s gap in clientside logs (see rageshake for details). This behaviour started at some point in the last month or so - perhaps when backup got added?

The regression hasn't been picked up by the perftest jig.

### Outcome

#### What did you expect?

Instant sync

#### What happened instead?

Sync regressing.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

433

### Homeserver

_No response_

### Will you send logs?

Yes