Where do I find an overview of what's going on in my system?

You will need to websearch for the process name to see what it does.

> performance issues

Open a terminal: ctrl+alt+t
Type:
top

To freeze (stop) the output use the key: q (or ctrl+c)

http://www.cyberciti.biz/faq/show-all-running-processes-in-linux/
http://www.cyberciti.biz/tips/top-linux-monitoring-tools.html

There is also htop.
http://www.debianadmin.com/display-top-cpu-processes-using-htop.html

Valgrind is useful to find out actual mem usage.
http://stackoverflow.com/questions/131303/linux-how-to-measure-actual-memory-usage-of-an-application-or-process
https://wiki.ubuntu.com/Valgrind

Index of debugging tools.
https://wiki.ubuntu.com/DebuggingProcedures

Natty kernel actually has some high cpu issue which is still under investigation. Bug #760131

Some background.
http://www.linux-tutorial.info/modules.php?name=MContent&pageid=83
http://www.linux-tutorial.info/modules.php?name=MContent&obj=glossary&term=process%20table
http://linuxgazette.net/133/saha.html
http://oreilly.com/catalog/linuxkernel/chapter/ch10.html

Forgot to mention conky, light weight system monitoring tool which is highly configurable.
After installation 'conky-all' package includes already a standard configuration file in /etc/conky/conky.conf.
e.g. one creates a file .conkyrc in /home/user, copy content of conky.conf into .conkyrc.
To run at startup it needs an entry in system settings -> startup applications, run cmd: conky-all.

http://packages.ubuntu.com/natty/conky-all
http://conky.sourceforge.net/variables.html

User examples of conky configuration.
http://ubuntuforums.org/showthread.php?t=281865

Thank you for the answers.

However, isn't there a blueprint or a global overview somewhere in the Ubuntu wiki or such?

> blueprint or a global overview

Log files actually tell what's going on.
https://help.ubuntu.com/community/LinuxLogFiles
http://www.cyberciti.biz/faq/linux-log-files-location-and-how-do-i-view-logs-files/

As you mentioned yourself, it's complex and modular, each process has it's cmd, which can be viewed in sys monitor by hovering over the process, then each cmd can be investigated for example by locating the string, package, dependencies, bug reports.

http://manpages.ubuntu.com/manpages/natty/man7/capabilities.7.html
http://www.tldp.org/HOWTO/HOWTO-INDEX/howtos.html