up to WP 3.3 having more than 50 or 100 pages in WP often slowed down the performance. It was recommended to choose an url beginning with a number.
Then it was fixed in 3.3, this post explains the details:
As it is very "technical" I don't undesrstand this solution completely.
Is the problem fixed now however I create my permalinks, with or without using the permalink tool of WP, whatever permalink setting in WP or manual creating/editing of the url-slug I choose? Or is there a case which is not fixed?