The down side is that this does not confer any immediate advantage on you.
In and of itself, running faster does not improve your quality of life.
What it does give you is a potential competitive advantage if you are
thinking faster than other people. So if the future world is one in which
competition is a significant factor, there will be pressure to run your
brain as fast as you can.
This is too bad, because actually in most ways the quality of life will
be worse for the super-fast brain. From the brain's point of view, it
is as though the rest of the world is slowed down by a huge factor.
Everything will be slow - computers, communication, any manipulation of
the physical world. If you need to do experiments to learn more about
nature, they will take (subjective) centuries to complete. If you want
to access those miraculous libraries from around the world, better plan
on (subjective) minutes or hours to get results. No more instant access.
If you expect to live in a nifty VR, think again: computers will be a
million times slower (subjectively) which will limit the quality of the
simulations they can provide. You may have to live in a cartoon world.
Some people look at this and say, "yeah, but I can get centuries of
work and thinking done in a single day!" I say, "wow, it will take
subjective centuries for even a single day's worth of change in the
physical world." I hope you like the weather, because you're going to
be stuck with it for a while.
Hal