It's not a fact about having a "slow search" It would probably not need more than a second. But the problem is that it may make the whole server respond a bit slower for everyone, causing more charge and maybe preveniting it to respond in time.
I don't think it's such drammatical, but I think you got me.
DZ:
I imagine you have all half-stickies defined on a separate table and that is linked by postid to the actual posts. Why don't you add a clumn in the posts that has 1 when the post is halfsticky or 0 when it isn't. This way you could easily order firstly by halfsticky bit and secondly by date. This wouldn't be much problem to the DB I think.