There might also be other reasons for this.
If for example you have a GROUP BY in your query then it might not perform an aditional sorting for ORDER BY ASC at all, because ascending sorting is implicitly performed on GROUP BY. But if you have ORDER BY DESC on top of GROUP BY it mut perform an additional sort, hence longer execution time.
Jurij Modic
ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?