Thanks for the report. https://github.com/ImageMagick/ImageMagick/issues/1819
doesn't mention performance problems, just corruption in the image. I can't reproduce your problem. The threshold tool works fine for me with or without conversion to PNG first. Please start gscan2pdf from the command line with the --log=log option, reproduce the problem, quit, and post the log file, which gscan2pdf should have compressed to log.xz.
signature.asc
Description: OpenPGP digital signature
