Help
It's probably true for me also...though I think needing a long time to think is often the sign of an unclear bid or situation not covered by bidding system/partnership agreement. So I would assume the better the player/partnership, the more likely thinking is actually productive, rather than wondering what p's last bid meant... 
It also causes stress, when they're thinking a long time on say a Blackwood response, you start to think "did they forget we agreed to rkcb and are trying to remember..." etc.
That said, I disagree to a limited extent with another poster's complaint about people who take too long to think when their bid is obvious/automatic. If it's truly automatic, then that is one of the few times in the bidding when one can pause to think, e.g. about a situation that may be coming up later in the bidding ("Will I double or bid 4♠ if opponents bid 4♥...") without giving one's p unauthorized information; think before the situation arises. Also, I try always to take a few seconds for any bid, even before passing, trying to keep all my bidding in tempo.
However, except for some relays, how many bids are truly automatic? Even Blackwood, isn't there at least a modicum of thought as to whether Blackwood or slam-invitational or to play (rare but sometimes retreat from p's 4-level suit bid) rkcb or normal, and if rkcb, what suit was agreed, etc. For transfers, it's possible to "super-accept" (bid one higher than necessary, e.g. 3♠ instead of 2♠) with a particularly good hand for that suit, etc. So even on a so-called "automatic" bid, some thinking may be required (and thus, there's a risk of giving UI).
