(All?) languages convey a mean of ~39 bits (+-5b/s) of information per second (source). Information is much more efficiently conveyed through writing.
I’m not sold on speed reading though. It seems, in practice, that you necessarily compromise comprehension, in the same way playing really fast a passage compromises accuracy (both muscle precission and auditive processing of sound, I’d say) if you don’t have a very deliberate, controlled sequence of movements.
Maybe in this domain there is something to learn from ultraslow practice, à la a golf player practicing their swing very slowly (as discussed in the context of music here).