The core explanation can be found back in the physics of our brain.
The brain's neural networks are made up of nerve cells that exchange pulses via synaptic connections. Unlike atoms in a crystal, which are arranged on a regular, cubic lattice, nerve cells grow synaptic connections in a highly specific but irregular fashion. The team of researchers -- Theo Geisel, Marc Timme, and Fred Wolf -- came up with a mathematical model which can precisely determine how fast neurons can coordinate their activity.
As could be expected, they found that the more highly connected the neural networks are, the faster the neurons can synchonize. But what was surprising was that this speed has an upper limit. Even in areas of the brain with the most dense neural networks, thought coordination can only happen so fast.
This "speed limit" on thought comes about because of the complicated switching structure of the neural networks. Only if every single neuron in the brain were connected with every other single neuron would the speed limit not apply.
— Physics Forum quoting Deutsche Welle.
So, the bottom line is that speed thinking comes with practice, training the brain improves connections.
But, there are all kinds of different things that arise when communicating which can slow you down:
Loss of focus, if you don't pay attention you won't be thinking towards an answer.
Lack of interest, which results in a lack of focus as you rather want to do something else.
Lack of knowledge, you can't talk properly about something you don't know enough about.
Again, communication skills come with practice; like learning more about the subject and talking more...
Personally, I think being introvert gives you a boost in speed thinking whereas being an extrovert gives you one in communication skills. Because introvert people think more while extrovert people participate more in a conversation. I think that a key step is that you could try to let that inner voice open up your mouth, you will automatically learn from the mistakes you say which might remain hidden if you only think about it.
As for reading, you should definitely check out the speed reading concept.
Speed reading is a collection of reading methods which attempt to increase rates of reading without greatly reducing comprehension or retention. Methods include chunking and eliminating subvocalization. No absolute distinct "normal" and "speed-reading" types of reading exist in practice, since all readers use some of the techniques used in speed reading (such as identifying words without focusing on each letter, not sounding out all words, not sub-vocalizing some phrases, or spending less time on some phrases than others, and skimming small sections).
Speed reading is characterized by an analysis of trade-offs between measures of speed and comprehension, recognizing that different types of reading call for different speed and comprehension rates, and that those rates may be improved with practice. The many available speed reading training programs include books, videos, software, and seminars.
Wikipedia - Speed Reading