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Definitely interesting. Even more impressive on their release notes of Tech Preview: https://github.com/owncloud/ocis/releases/tag/v1.0.0

It's also a smart move because I'd be a willing to bet Go is more approchable to would be contributors.

Is this going to make migrating from NextCloud worthwhile for security or speed reasons?

I'd probably consider it if it was faster and lighter on resources.