Guess it would be much more performant, if you convert your PDF to TIFF first using gdal_translate inside OSGeo4W shell.
With transparency:
gdal_translate --config GDAL_PDF_BANDS 4 --config GDAL_CACHEMAX 1024 -co NUM_THREADS=ALL_CPUS -co COMPRESS=DEFLATE -co ZLEVEL=9 -co PREDICTOR=2 -co TILED=YES <source.pdf> <target.tif>
Without transparency:
gdal_translate --config GDAL_CACHEMAX 1024 -co NUM_THREADS=ALL_CPUS -co COMPRESS=JPEG -co PHOTOMETRIC=YCBCR -co TILED=YES -co JPEG_QUALITY=90 -b 1 -b 2 -b 3 <source.pdf> <target.tif>
Afterwards you can add overlays as well:
gdaladdo --config COMPRESS_OVERVIEW JPEG --config PHOTOMETRIC_OVERVIEW YCBCR --config GDAL_TIFF_INTERNAL_MASK YES -r cubicspline <source.tif> 2 4 8 16