Slow transactions

At high load conditions the server app stops responding to client because begin/commit statements are executing very slowly (some take 15 seconds). The queries are simple - insert, update two columns on one record by id, select a,b,c from d order by limit 50, etc...

pgAdmin shows a lot of locks and queries like

 set extra_float_digits=3; set ssl_recognitation_limit=0; select 'npgsql12345';
 DISCARD ALL
 COMMIT
 BEGIN; SET TRANSACTION ISOLATION LEVEL READ COMMITED;

There are ~5 actively used tables, some of them have up to 200k rows.

The server app uses NHibernate with Npgsql.

VACUUM/ANALYZE didn't help.

When can be the cause of the slowdown?