pg_exec commit causes extremely long delays

I have a TCL app which has severe delays - measured in tens of minutes at 
least - running "pg_exec $conn "commit".

The delays are so long that I used to think the app was hopelessly frozen. 
By accident, I left the app alone in its frozen state and came back a good 
eal later and seen that it was running again. The application loops through 
an import file, reading one row at a time, and issues a bunch of inserts and 
updates to various tables. There's a simple pg_exec $conn "start 
transaction" at the beginning of the loop and the commit at the end.

There are a minimum of four apps running at the same time, all reading 
different sections of the same table, all writing to the same db and the 
same tables. The other apps seem unaffected by the one app that freezes.

Any thoughts on the cause and possible solutions would be appreciated.

Carlo