Do you have a source on the speed limit of biological computation. Potential gradients should behave just like electricity. Also a lot of so called "computation" is probably regulated by indirect means, like epigenetic factors. It's definitely more than a bunch of neurons messaging each other. Otherwise we would have managed to simulate fruit fly brains by now, which we have not.

Its just a misunderstanding. All forces take place at lightspeed. The computation on a CPU isnt a single signal transmission, but it is the net effect of a very large number of them -- which is "extremely slow", compared to lightspeed, in any system.

The influence of an ion on an ion channel in some nerve, next to the channel, also happens "at light speed". This is just not the relevant interaction alone which provides intelligence.