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.