I create quite big illustrations with a lot of small complex symbols/icons in it, for example different kinds of servers etc. Some icons/symbols are used multiple times in one illustration. Would it improve the performance of illustrator if I convert those icons to illustrator-symbols and use those ones? From my point of view illustrator would only have to save the location of a reference to a symbol instead of all the details of the icon...
I'd imagine so, but whether you'd notice the difference is another matter. Sounds like you should be using symbols anyway given your use case.
In your case, possibly a little bit.
I use symbols extensively when creating layouts for GUI and I will convert bitmap screenshots and Illustrator objects to symbols for ease of updating dozens of instances. I find it does improve redraw a little for me, but not to the point where I would automatically turn every unique object to a symbol.