ImageMagick convert -geometry weirdness

I don't know if this is really documented anywhere, but when using the ImageMagick program convert with the -geometry or -resize flags it also converts to 16bit color, at least when used against 1bit (Black and White) image.

The reason this is important to note is that using -compress Group4 will not work on these images because they aren't 1bit color images and they need to be. To work around this limitation you need to force the color depth with -monochrome. The downside to all of this is time.

Ripping an Image file from a PDF takes roughly .206 seconds for an 8.5x11 PDF with pdfimages (useful if a PDF has OCR embedded in it.)
Converting the resultant PBM file without -geometry 1700x2200! -monochrome takes roughly .194 seconds.
Converting the resultant PBM file with -geometry 1700x2200! -monochrome takes roughly 4.594 seconds. This is an increase of 10 - 10.5 % adding in the geometry.

*all of the above numbers are from one file, but testing of different files showed similar results. All tests where done at 200 DPI.

Here are some examples of the commands we are running for anyone curious.
convert ${outputTiffRoot}-000.p*m -density 200 -compress Group4 {outputTiffRoot}_200.tiff

convert ${outputTiffRoot}-000.p*m -density 300 -compress Group4 {outputTiffRoot}_200.tiff

convert ${outputTiffRoot}-000.p*m -density 200 -compress Group4 -geometry 1700x2200! -monochrome ${outputTiffRoot}_200.tiff

convert ${outputTiffRoot}-000.p*m -density 300 -compress Group4 -geometry 2550x3300! -monochrome ${outputTiffRoot}_300.tiff