IIRC the ten thousand hours thing was ten thousand hours of tutored practice at a level appropriate to the learner

The important factor is that it is deliberate practice. Tutors are useful but not as necessary during the practice (this obviously varies depending on the degree and kind of feedback required).

I can see Clark running into the limitations of other people's performance rather than his own as a bottleneck.

In particular where the information is not yet contained in all the textbooks and internet resources currently in existence his learning will be much slower. He'll have to invent the science (or engineering) himself as he goes.

Assuming that the limit is in the PS/2 protocol (and not in the keyboard hardware - Clark may have quietly replaced the keyboard on his desktop with a high-speed variant that he'd built himself, but it still needs to talk to the computer using a known protocol);

My recently primed munchkin instinct can't help but notice that the analysis given doesn't remotely approach the limits specified here. Specifically, it tacitly assumes that Clark uses only the stock standard software that everyone else uses. In fact, it even assumes that Clark doesn't use even the most rudimentary macro or autocomplete features built in to standard wordpressors!

Assuming that at some point in his life Clark spent several minutes coding (at the limits you calculate) in anticipation of at some point in the future wishing to type fast all subsequent text input via the PS/2 protocol could occur a couple of orders of magnitude faster. Optimisations would include:

• Abandon the preconception that pressing the key with the "A" painted on it puts the letter 'a' in the text, or any of the other keys for that matter---especially the ones that aren't so common! Every key press is log2(number of keys) bits of information. Use all of it.
• A key_press uses 33 bits of bandwidth total but key_press isn't a discrete operation. 11 bits are used for key_down and 22 for key_up but these don't need to follow each other directly (for example see conventional usage of shift, control and alt). As far as the PS/2 protocol is concerned key_up supplies another log2(number of keys) bits of information (for the cost of 22 bits of bandwidth).
  • Given that Clark constructed his own hardware he could easily make use of the full 2*log2(number of keys) bits of information per 33 bits of information by making his keyboard send only a key_down on the first keypress and a key_up on the second keypress (alternating).
  • If Clark is using a standard keyboard then he can still send more information via key_up but is now limited by fingers. Since he has only 10 fingers, before every keydown (after the first 10) he can send one or more key_ups. Which finger(s) he choses to lift up is influenced by the proximity of the keys to each other. Optimal use of this additional information would use a custom weighted "twister protocol" that extracts every bit of information available in the choice "left index finger T" instead of "right pointer T" when both were bio-mechanically plausible options. For this reason, if Clark is using a standard keyboard I recommend he use the smallest layout possible. A laptop's keys being cramped is a feature!
• Human languages (like English) are grossly inefficient in terms of symbol use. Shannon (of shannon entropy fame) measured the entropy of English text at between 1 and 1.5 bits per letter even when using mere human subjects guessing what the next letter would be. Some letters are used way too much, simple combinations of letters like "atbyl" have no meaning, some words combinations are more likely than others andIcanreadthiswithoutdifficulty. If bandwidth rather than processing power is the limit compression is called for. I estimate that Clark's Text Over PS/2 Protocol ought to be at least as efficient as Shannon's "subjects can guess what is coming next" findings for typical text while remaining lossless (albeit less efficient) even under unusual input.
• Since Clark wants to maintain a secret identity his keyboard must be required to operate normally except when he is typing fast. This is easy enough to accomplish via any one of:
  • An unmarked button that requires superhuman strength to press.
  • A keyboard combination (F12 D u _ @ F3 W * & etc) that will not occur randomly but still takes negligible time to enter.
  • The software just starts interpreting the input differently once a sufficient number of keys have been input in rapid succession. (This seems preferable.)

Anyhow, there is a limit to the speed at which even Superman can type; that limit being the keyboard. Your average keyboard is more than fast enough to keep up with a human typist, but not infinitely fast...

Samantha Carter used an entire computer lab when she was supercharged. Her limit was the key buffer if I recall. Depending on how computers have evolved the limiting factor could be the mechanics these days. If so, then it may be efficient to use several keyboards simultaneously.

Clark could certainly work faster than that if he were, say, engraving on a stone tablet

Especially if he uses his (laser) eyes. Depending on his power level he could possibly write faster than the speed of light. If my past research is correct superman is the most powerful when inside a sun (a blue star is best but ours would be fine). So he could perhaps write the most quickly by positioning himself at the surface and the sun and engraving on the surface of say, Mars, or perhaps a moon of Jupiter. The limit of is output then be either the precision of his eyesight, how fast he can control the muscles that move said eyes or, if those capabilities are sufficiently excessive, how fast he can think.

I dunno, how fast can a chisel carve stone before getting blunt, shattering the tablet, or something?