Tracing large memory usage and slow response time - looking for additional hints

Hello

I have following problem:
Something (Maybe some module ?) on my portal is consuming large amount of memory
and for some reasons the response times for simple pages are very slow.

I tried to narrow it down using devel and trace modules, but it didn't help.

Devel output shows:

Page execution time was 2021.95 ms. Executed 591 queries in 93.71 milliseconds.
Memory usage:

* Memory used at devel_init(): 79.83 MB
* Memory used at devel_shutdown(): 97.55 MB

Nearly 80 MB of memory were used by something before the request has been started at all.
How can I identify what is it ?

On one other very simply Drupal installation I get following output:

Page execution time was 549.31 ms. Executed 117 queries in 94.14 milliseconds.
Memory usage:
Memory used at devel_init(): 0.7 MB
Memory used at devel_shutdown(): 1.37 MB

So only 0.7 MB

Similiar thing when I am using trace.

On the complex installation:

#210cc3df T=0.000000 [REQUEST] 2009-01-16 18:21:20.837796 GET /community HTTP/1.1
#210cc3df T+0.315741 [NOTICE ] E_NOTICE: Constant PREG_CLASS_PUNCTUATION already defined in `modules/search/search.module':83

The "NOTICE" is the very first entry in trace log after the "REQUEST"
How can find out what happens in the time period of 0.315 sec inbetween ?

In the simply installation in the same time I (0.315 sec ) get already the first "RESPONSE"

Has somebody any hints how can find out what consumes the time and memory
in the complex installation ?.

Details:

Complex installation:

Drupal 5.8
206 ative modules
Server has two CPUs and 4 GB RAM

CAPTCHA Already 0 blocked form submissions
CURL INSTALLED
Configuration file Protected
Database schema Up to date

Mime type detection Browser & Extension
MimeDetect is using the browser supplied mime type or file extension lookups. It is strongly recommended that you install and configure the PHP Fileinfo Extension or the UNIX 'file' command to provide more accurate severside mime type detection.

MySQL database 5.0.32 MySQL database for event module 5.0.32
PHP 5.2.0-8+etch13
PHP 5.2.0-8+etch13
PHP register globals Disabled
Unicode library PHP Mbstring Extension
Web server Apache/2.2.3 (Debian) PHP/5.2.0-8+etch13
jQuery Update Installed correctly
The current installed version of jQuery is 1.2.6

Cachinng is enabled in normal mode
CSS aggregation is enabled

Simply Installation:

Drupal 5.10

few Modules active (core, devel, tracing and few others)
Server has one CPU and 256 MB RAM

Database schema Up to date
GD-Bibliothek 2.0 or higher
Configuration file Protected
MySQL-Datenbank 4.1.10a
PHP 4.3.10
PHP Register Globals Disabled
Unicode-Bibliothek PHP mbstring-Erweiterung
Webserver Apache/2.0.53 (Linux/SUSE)