QuickFIX is considerably slower than commercial FIX engines. The fastest one I have seen is capable of about 17000 messages a second, written in highly optimized C. A very good one can do about 11000 messages a second.
Other than the FIX engine layer, equally important is the transport layer. If you have a fast transport layer but it cannot deal with possible queuing issues, it could be a very bad experience.
That said, for 99% of the people that hang out on ET, it is a perfectly reasonable FIX library. I have used it and executed thousands of ATS generated trades with it without a hitch. Search for nitro and quickfix for more opinions.
The people that shouldn't use it won't be posting on ET, so I won't go there.
nitro