[imp] IMP Performance in mailbox.php

[Johny]

I've done some tests using imapproxy (see separate mails to that list 
for results) to find out whether it is worth using or not (the short 
answer for my test case is no, it slows things right down!)

While I was at it I added some timings to the main file in IMP, 
mailbox.php, and was astounded at the results. This might be well 
understood, so I'll summarise and those of you working with IMP and PHP 
more can comment on the results.

The tests are against IMP 3.2.1, I tried upgrading my server to run with 
the HEAD, but too many problems encountered so I backed out and will get 
it working on my desktop instead for dev purpose some other time.

Displaying 20 mails with UW-Imap as the server;

Total elapsed time - 1.55 seconds

Split up, all times are elapsed from start of mailbox.php;
To get past all the require_once statements - 0.81 seconds (over HALF!)
To get past 'IMP::checkAuthentication();'   - 1.34 seconds

(I have more detailed timings, but these two are the main culprits for time)

These are well before the code written in mailbox.php is executed. The 
actual IMP code takes around 25ms or so by my estimate to run, out of a 
total execution time of 1.55seconds.

I realise PHP is interpreted with the costs that incurs, and also that 
the issues I raise may have been rectified in either later PHP versions 
of mailbox.php, but as I likely won't be looking at this for a week or 
so again I thought I'd let you know.

Cheers,

Johny