Poor Performance When Calling Query From Another DB on same Server
The issue was the Compatibility Level as suggested by @Kin 

One database was 120, the other 100.

I updated to ensure both are 120 and now queries are quick on both databases!

https://docs.microsoft.com/en-us/sql/relational-databases/databases/view-or-change-the-compatibility-level-of-a-database

Thanks all for the help!