We're experiencing an intermittent problem whereby the
logsegment in tempdb gets <40% space free and a threshold
procedure is fired off. The proc collects some information
about what was running at the time then dumps the
transaction log with truncate_only. This regularly takes
around 30 mins to complete. While the dump is in progress,
the log continues to grow and further thresholds are passed.

We're unable to figure out why it takes this long, as we're
just truncating the log not saving it to an O/S file. Any
ideas would be much appreciated.