I don't like boasting, but I would say that I would qualify as somewhat of a fast calculator.
I can rapidly mentally divide extremely long numbers by some digit (like 3), up to many decimal points.
Also, I can mentally extract square roots of perfect squares that are up to 4 digit long. I think I did some square root extractions of 6 digit long numbers, but that was rather difficult.
This one isn't really fast mental math, but I can also rapidly memorize a very big amount of digits in a short period of time. My record was memorizing 100 digits in just 15 minutes. (I can memorize up to as many digits as I want provided that I have enough time - I haven't found a limit for that yet.)
Also, wanted to add that I don't exactly have a mild Autism Spectrum Disorder. In my case, it affects language and communication to some extent, and the last tests I had showed very huge discrepancies between different cognitive abilities - some were literally absent, some were very developed. For example, ironically, I was unable to solve any (not even one!) arithmetic problems that were presented verbally during the testing, despite boasting a rather good GPA in math and very good mental math skills - the verbal information during the test simply overwhelmed my sensory system.