Welcome to the community Simon!
MongoDB 4.3 is the development release series leading up to the 4.4 production release (see MongoDB Versioning). You can download development/unstable releases for testing, but we do not recommend running them in a production environment as they will include work in progress and have not been throughly tested yet.
MongoDB 4.4 is currently in the “release candidate” stage of testing, where there will be several 4.4.0 release candidates available before a final Generally Available (GA) release. Release candidates are also for testing purposes only, but you could test in a development/staging environment to confirm if your issue will be resolved.
The specific issue you have highlighted is not a general fix for “sorts running very slowly”, so I recommend starting a new topic to explore why your aggregation pipelines are slow.
It would be helpful to include:
- your current MongoDB server version
- your deployment type (standalone, replica set, or sharded cluster)
- aggregation pipeline and explain() output
Regards,
Stennie