Irrlicht has 2 software renderers: one is fast but very buggy (on my computer at least), and the other works well but is too slow. The slower one uses a kind of bilinear/trilinear filtering on the textures, and I guess it could run much faster with this filtering turned off. Where could I disable it in the code?
You can change the compile-time speed settings either in IrrCompileConfig.h or you can set a custom profile in SoftwareDriver2_compile_config.h (undefine SOFTWARE_DRIVER_2_BILINEAR)
Currently SOFTWARE_DRIVER_2_USE_WBUFFER must be defined, which I'll raise as a bug on the tracker shortly.