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This release fixes a regression introduced in Synapse 1.49.0 which could cause `/sync` requests to take significantly longer. This would particularly affect "initial" syncs for users participating in a large number of rooms, and in extreme cases, could make it impossible for such users to log in on a new client.

**Note:** in line with our [deprecation policy](https://matrix-org.github.io/synapse/latest/deprecation_policy.html) for platform dependencies, this will be the last release to support Python 3.6 and PostgreSQL 9.6, both of which have now reached upstream end-of-life. Synapse will require Python 3.7+ and PostgreSQL 10.

**Note:** We will also stop producing packages for Ubuntu 18.04 (Bionic Beaver) after this release, as it uses Python 3.6.

Bugfixes
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- Fix a performance regression in `/sync` handling, introduced in 1.49.0. ([\#11583](https://github.com/matrix-org/synapse/issues/11583))