stupid mysql question
 

What happens with my database performance if i define a field as TEXT
although 90% of the entries will be used for small messages (for which varchar would be most suitable) ?

The performance will be all good but they will decrease with the data since text take more memory. Not some thing that you should feel. :)

Ok.. but lets say.. i store 10.000 records with the word "hello" in a text field
and i store 10.000 records with the word "hello" in a varchar field on some other database.. no problem there?

The only issue i can think of is that for example a varcahr(4) reserves actual
4 bytes of memory while a 4 character text field has a default of 64 bytes or something.. whether you use it or use less.. That would kill performance in the long run.

It doesn take memory space it takes diskspace