Carl-Adam, had you pre-heated the buffer cache before running your
test. If not, could you please do so and rerun it? I just can't
imagine even .net would be _that_ slow.

I have no idea about what you are talking about :/ I am not much of an experienced programmer. But I knew it would be very slow, and I'd never choose C# or .NET for perfomance tasks. The main purpose was really to give a boost to this discussion about regex, and it worked.

The System.IO.StreamReader that I used does not use any buffer (nor cache), to my knowledge. I can see what happens if I use a buffered stream :)

Besides, Mono itself is written in C# and for the .NET framework, so in reality it has to compile iself at runtime (on-the-fly) aswell as for my application. Not sure about Microsoft's framework, but if I've got it right the .NET framework are mainly wrappers of the old .dll's and libraries and so here goes the microsoft problems again: layers, layers and even more layers before the work is actually done.
While on Linux (and other *nix) the libraries often work directly with the kernel. But this is no news.