mencoder very slow on windows

I compiled mplayer with xvid support using mingw, and I tried to encode a 
video to xvid with mencoder. I realized that mencoder is much much slower 
than on linux (the estimated time to finish the encode was about 60 minutes, 
while on linux it takes about 20 minutes). That was on the same computer, 
using similar videos, and exactly the same options.

That's even without the --enable-runtime-cpudetection option, so the binaries 
are supposedly optimized for my cpu.

I realized of this warning when compiling xvidcore, I don't know if it has 
something to do:

checking for nasm... yes
checking for nasm patch version... nasm: error: unrecognised option `-r'
nasm: error: no input file specified
type `nasm -h' for help
-1
configure: WARNING: nasm patch version is too old
configure: WARNING: no correct assembler was found - Compiling generic sources 
only

I don't understand. I compiled the latest available version of nasm.

-- 
RVM