Views created by SQL server Database Tuning advisor Several years ago I ran Database Tuning Advisor on our database. Then the database wasn't very big so it was easy.

I implemented the suggestions, and it created several views with names like _dta_mv_xxx

I have never written a query against those views, and I wonder if they are used in any way by SQL-Server. I can't see any indexes on the views either.

Is it safe to delete those views or will it impact performance?

The database is around 400 gb so working with creating and recreating indexes is slow.

The database is SQL Server 2016 Enterprise.
The short answer is that it's impossible to tell, but it's not terribly likely.

Views without indexes are just obfuscated queries. Unless you've written queries directly against those views, there's generally no advantage for the optimizer to decide to use them.

The safest thing to do would be to


Script their definitions 
Script the DROP commands you plan to use
Check them into change management or source control


That way, if you drop any, and something odd happens (like queries failing), you can add them back. If you have a dev/QA environment, make the changes there first and let them burn in for a week.

I'm not sure why you brought up index creation speed. If you're under the impression that creating a view that isn't indexed requires some physical structure gets built, it doesn't.

On the flip side, the views created by DTA are often created with the SCHEMABINDING option, which makes altering the underlying tables and columns that they touch throw errors that are confusing to track down.

Hope this helps!