I'm a native English speaker, I don't think I would ever say "two times bigger", sounds like bad grammar. I would read "2x the size" as "twice the size" or "two

I'm a native English speaker, I don't think I would ever say "two times bigger", sounds like bad grammar. I would read "2x the size" as "twice the size" or "two times the size", but not "add on twice the size". So I agree with the article, "3.5 times faster (1 hour vs. 3.5 hours)" is perfectly correct, and it is OK to abbreviate "3.5 times" as "3.5x".