Problem with performance when using Stored Routines i.c.w. ORDER BY or Temp Table

I already posted my question on Stackoverflow:
[URL]performance - MySQL query slow because of ORDER BY with Stored Functions - Stack Overflow

The problem appears when I try to use the results of the Stored functions in the ORDER BY. I also tried to store the result (without the ORDER BY) in to a temporary table, but ended up with the same result. The query runs fast if I remove the ORDER BY.

Any directions are very welcome, thanks!