It seems to be started in a different file to that you have listed.

SessionHandler/Storage/Stack.php

The read method in that file that i can get a clear time reading that 
shows it locking up.
Ive worked out that there are two storage objects, memcached & sql.
The time seems to be sticking on the Memcached driver.

SessionHandler/Storage/Memcache.php # line 91 - $this->_memcache->lock($id);

This shouldn't happen. Can you track it further down into 
SessionHandler/Storage/Sql.php inside the read() method? Is it 
transactionStarted(), beginDbTransaction() or selectValue()?

Changes have been made in Git (develop):

commit 788a5259f75d697d4bab5cd3c69f7660176a9316
Author: Jan Schneider <jan@horde.org>
Date:   Fri Feb 10 18:51:27 2012 +0100

     [jan] Don't start transaction more than once in SQL driver (Bug #10978).

  .../lib/Horde/SessionHandler/Storage/Sql.php       |    4 +++-
  framework/SessionHandler/package.xml               |    4 ++--
  2 files changed, 5 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/788a5259f75d697d4bab5cd3c69f7660176a9316

Ive narrowed down the problem.

Horde/SessionHandler.php:177
It will hang on this call though to the DB read call but only if 
userid is equal to the currently logged on userid. This was confimed 
by putting a continue on a loop if id matched session_id;
So maybe it seems that you are still holding a row readlock open on 
the session table of the user.

It's still the wrong patch. You need to find out where it's spending 
the additional time.

Thanks put putting the patch in the correct place but it seems it does 
not fix the problem very well.

While your patch does stop the error from being created the page load 
time is very very slow.
Without my patch the sessions page takes 9 seconds to open up, with my 
patch it takes under 1 second to open up.

Changes have been made in Git (master):

commit 788a5259f75d697d4bab5cd3c69f7660176a9316
Author: Jan Schneider <jan@horde.org>
Date:   Fri Feb 10 18:51:27 2012 +0100

     [jan] Don't start transaction more than once in SQL driver (Bug #10978).

  .../lib/Horde/SessionHandler/Storage/Sql.php       |    4 +++-
  framework/SessionHandler/package.xml               |    4 ++--
  2 files changed, 5 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/788a5259f75d697d4bab5cd3c69f7660176a9316

This is exactly what's the driver is supposed to do. The transaction 
is commited when writing the session information at the request's end.

When we view the Administrations->Sessions the page comes up with an error.

--horde.log--
2012-02-10T12:58:08+00:00 EMERG: HORDE [horde] There is already an 
active transaction [pid 28447 on line 644 of 
"/home/httpd/live/webmail.wenn.com/htdocs/pear/php/Horde/Db/Adapter/Base.php"]

--weboutput--
A fatal error has occurred
There is already an active transaction
1. Horde_SessionHandler->getSessionsInfo() 
/home/httpd/live/webmail/htdocs/admin/sessions.php:29
2. Horde_SessionHandler->read() 
/home/httpd/live/webmail/htdocs/pear/php/Horde/SessionHandler.php:275
3. Horde_SessionHandler_Storage_Stack->read() 
/home/httpd/live/webmail/htdocs/pear/php/Horde/SessionHandler.php:177
4. Horde_SessionHandler_Storage_Sql->read() 
/home/httpd/live/webmail/htdocs/pear/php/Horde/SessionHandler/Storage/Stack.php:88
5. Horde_Db_Adapter_Base->beginDbTransaction() 
/home/httpd/live/webmail/htdocs/pear/php/Horde/SessionHandler/Storage/Sql.php:91
6. PDO->beginTransaction() 
/home/httpd/live/webmail.wenn.com/htdocs/pear/php/Horde/Db/Adapter/Base.php:644

This error does not happen straight away after you login, it happens 
after you have gone though a few pages.
The issue does not seem to happen if i use the MySQLi driver instead 
of the PDO.

Things to note, we are running horde sessions in MySQL due to having a 
HAproxy LB setup.

Ive attached a patch file to fix. Seems that you are not closing the 
transaction just after you retrieve all the the SessionIDs, afterwards 
you then have to do lots more reads.