Why does a query slow down drastically if in the WHERE clause a constant is replaced by a parameter (having the same value)?

Question

I have a recursive query which executes very fast if the WHERE clause contains a constant but becomes very slow if I replace the constant with a parameter havin

Answer 1

This could be the worse suggestion ever, but have you considered creating a sproc to create your query as a string and execute it using sp_executesql?

I know nothing about the caching behaviour of SQL executed by sp_executesql, it was just the first thing to pop into my head.