App can be slow to incremental sync

[manuroe]

As per the log below on 1.6.1.
The server response was slow (this is a known cache expiration issue in synapse) but the app tried to sync only after 6 seconds.

# First log - App starts
2021-10-03 09:55:54.233 Riot[65448:5771531]  [MXLogger] redirectNSLogToFiles: YES

# First sync request
2021-10-03 09:56:01.312 Riot[65448:5771531]  [MXHTTPClient] #7 - GET _matrix/client/r0/sync

# The slow response from the homeserver
2021-10-03 09:56:08.056 Riot[65448:5771531]  [MXHTTPClient] #7 - GET _matrix/client/r0/sync completed in 6743ms

# Total time to remove the grey spinner
2021-10-03 09:56:09.867 Riot[65448:5771531]  [MXSession] Events stream resumed in 8555ms
2021-10-03 09:56:09.867 Riot[65448:5771531]  [MXKAccount] @Manu:matrix.org: The session is ready. Matrix SDK session has been started in 15351ms.

[MadLittleMods]

Related to #5752