No, it's actually much worse.

I tested my function against 16540 row table.

It returned result in 3..5 seconds (I'm not alone on that server )

Than I started same query but using function [dbo].[CountDP].

I's been 2 hours 50 minutes since then, it's still going.

So, there is a reminder: avoid referencing tables inside UDF!

Even if it's such "set based" table as Numbers.