Why? Except Java apps are slow (at least in my experience).Because the Java language, in a word, sucks.
I'm not pronouncing it here.It's called Qt, not QT. And it's pronounced "Cute".
Sadly (or not), every C# application I used is slow. People complain about F-Spot, Tomboy, because they're slow. Not only runtime is slow. If the Garbage Collector is a mess in Mono (and afaik it is) then there isn't a big chance to optimize the code, is there?It won't necessarily be slower. With a more modern language, you spend less time debugging stupid bugs than with C, which allows you to spend more time on optimization (or new features). This even applies if your preferred language is too slow for some part of the program: you can implement that part in C and do all the rest in a more productive language. Well, except that if you're using Java, you don't really what to do that, given the horrible PITA that JNI is.
In this thread there's a nice picture why some (probably many) users don't like Mono: