From the Autolite Site
"Autolite Heat Ranges - The larger the last digit in the part number, the hotter the plug is. Example a AR3935 (good street/strip plug) is hotter than a AR3932 plug which is very cold used for lots of nitrous."
AR3910X is about the coldest plug on the planet. I bolded and underlined 'very cold.'
The 3910X is 2 steps colder than the above listed 3932.
I think the only reason the 3910X became popular is for the extremely minor amount of compression increase due to the
electrode design.