If the fibre is correctly lubricated it will not be scorched , Like I said I have been using this method for 17 years with zero complaints of this nature . I have seen plenty of shrunk carpets (bw) due to use of extraction machines. I currently use a twin speed WETROK , with a victor sprite as a back up.
Slow speed being necessary for bonnet buffing is a big myth in my opinion, thousands of carpet cleaners in America use this method. The benefits of speed ,by far outweigh the possible down sides.