In our SQL Server Database we have at least 25-35 lookup / empty tables that are never used. Is there any negative effect for keeping them in our database (performance, storage etc)
Basically, would deleteing them have any positive impact on queries etc.
No there would be no positive or negative impact to removing or leaving them there, if they are not used. I would remove them.
If queries don't reference the tables, then there would be no impact to those queries. SQL Server will pay attention to frequently requested information and cache that information in memory. Having a table out there should result in no notable difference in performance or storage.
Leaving them couldn't have any negative impact that I can imagine.
If you're never referencing them, they'll never get loaded into memory, so they won't affect performance. As far as storage goes, since they're lookup tables, I would assume they're small. These lookup/empties will have minimal impact on storage.
I would removed them to have a clean database. It is a good practice to remove all unused objects. Otherwise you will end up with tons of structure that is not in use anymore and noone knows.
The performance and storage is irrelevant.
Remove them anyway - it's a pain to maintain a database when you have to first figure out what is and isn't being used.