There is no downgrade path. It's just a question of migrating your data structures and data down to the older version. Personally, I wouldn't do it.

If you're seeing performance issues, there's a good chance that's primarily in what would be considered edge cases. Instead of downgrading, I'd suggest tuning the queries and structures so that they perform better. There's not a massive known improvement in performance between most SQL Server versions. Rather it's incremental along with additions to behavior with the newer versions.