Major regression in sync time

[ara4n]

Steps to reproduce

For the last ~2 weeks, EXI reliably takes ~30s to sync for me on every fresh launch.

Doug looked into it, and saw:

The first /sync request is instant but fails because the token need to be refreshed.
It takes 1.5 seconds to refresh the token.
The second /sync request is sent and 2 seconds later a SyncLoopSkipOverCurrentIteration happens.
No response is received.
A third /sync request is sent and 7 seconds later another SyncLoopSkipOverCurrentIteration happens.
No response is received.
A fourth /sync request is sent and takes 34 seconds until a response is received.
Then there's 3 more SyncLoopSkipOverCurrentIteration over 10 seconds before everything starts to work as normal.

Filing this issue to try to get it properly triaged and fixed.

Outcome

What did you expect?

Instant sync

What happened instead?

Worse sync than Element Classic.

Your phone model

iPhone 12 Pro Max

Operating system version

iOS 26

Application version

1062

Homeserver

matrix.org

Will you send logs?

Yes

[Hywan]

A SyncLoopSkipOverCurrentIteration can happen when:

1. we subscribe to new rooms (so basically when the user scrolls),
2. when a list mode changes (so when the state machine inside the room list is updated).

That's it. So I think the SyncLoopSkipOverCurrentIteration you get is because you're scrolling the room list. Which is not a problem at all. The problem is the server takes forever to reply. Can we raise this issue on Synapse maybe?

[bahur142]

It is the same for me with the new version 25.11.1 (195), iOS 18.7.2

[ara4n]

cc @AmandineLP who is also suffering this

[ara4n]

element-hq/synapse#19175 is the synapse issue for bad sync perf, and element-hq/synapse#19206 is the fix. i guess we see if it all goes away once the fix lands.