Do large threads cause load?

[Bilderback]

I've been getting alot of SQL locks causing long cpu times for a query.
The thread in question happens to be 710 posts in 72 pages.

used  35.00 seconds of cpu time for [[mysql query]]
# Sun Jan 18 11:24:44 2009
# Query_time: 33  Lock_time: 0  Rows_sent: 1  Rows_examined: 717
SELECT postid FROM post WHERE threadid=1155 ORDER BY dateline LIMIT 1

Is this normal for long threads or could something else be causing that particular thread to crash?
Is there something I could add or enable such as a cache system or mod?
Thanks all