Fixing missing room summary last messages slows down the initial sync

### Steps to reproduce

After this [PR](https://github.com/matrix-org/matrix-ios-sdk/pull/1096), last year, an initial sync required 467 decryptions on a 600 room account, using 2.8s of an iPhoneX CPU.

1.8.17 requires now 4643 decryptions for a 850 room account. They use 120s of iPhone 13 CPU before this fix for https://github.com/vector-im/element-ios/issues/6239 (Maybe 25s now).

But we are still comparing 2.8s of iPhoneX with 25s of iPhone 13.

The change comes from https://github.com/matrix-org/matrix-ios-sdk/pull/1206/commits/1f9d638863ad6666a5b131c8a8a4acfa7f1c59be?diff=split&w=1 that transforms the lazy fix of room summaries by a mandatory step.

This step is expansive. It fetches (and decrypts) 50 messages per problematic rooms.

### Outcome

Reduce the number of required decryptions during an initial sync.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No