What should I ask my DBA in order to diagnose a slow application?
While a good DBA can find out a lot on his own, it never hurts to give a lot of information.


Which application you're using. If you know server/db ect give that as well.
Test/Prod
When the issue occurs, eg. I open this form and it's slow, or when I do this...
What's slow and what's fast. For some applications it can be perfectly acceptable to wait 1-2 seconds, while others have to perform near instantly. It can often be really easy to get a first bump in performance, but after that it takes a lot longer to make it just a little bit faster
Which login you're using  


Especially if he's external + perhaps a junior it never hurts to just give him all you've got.

After that it's mostly up to the DBA to setup monitoring.


Log WhoIsActive to a table
Use the build in query store if version is 2016 or higher
Buy a third party tool, eg. SentryOne/Red gate monitor


After he setup monitoring you could also mail him "I'm having issues right now, could you see if you notice anything special?" Do not expect him to reply right away, but it gives him a window that he can check later where he knows the issue occurred.