Hi Colum,
There are two part to this question
1. Slowness - We need to find out where the actual slowness is, it could be slow at server side, it could be due to the custom domain design.
2. Setting the default, there is something called pre filter where you can filter the data but and in the filter you can set value as default but thats only after creating the report and not at run time while creation of report.
Note: Adhoc report doesn't fit the bill of creating complex logic and for that Studio reports are preferred. Hope this helps.
Regards
Suman Pramanik