There are fast opto-couplers designed for logic signals and supporting baud rates of 5M or so, but
they are more expensive and require logic power on the receiving side. For instance the ASCL6400 family.
There are also non-opto isolated logic couplers now that use capacitive coupling on a GHz carrier,
these handle 10MHz or so logic rates and use less power than most opto-couplers.