|This is a GREAT question, and it is SOLUTION Dependent!
If you could make the GNU Compilers twice as fast, PLEASE DO. Every time!
Well, until the time savings is so tiny, nobody cares.
We have a phrase: "The right answer late... Is the wrong answer!"
To us, it means that timing is part of the design/interface.
A Ping response that is normally > network timeout is useless to the recipient, as they gave up!
A page that takes 1 second longer than a user will wait, is too slow.
Sometimes, it's about "feedback" (Working... Spinners, Changing Screens, see Travelocity, LOL)
Sometimes, it's simply too slow to feel useable. Or I can do it by hand.
When I can launch EXCEL faster than find my calculator... You can bet I will use excel.
Therefore, either FIND a situation that REQUIRES twice as fast JSON parsing (Apps/Databases/etc), or move on.
Personally, I won't optimize beyond "acceptable" without a business requirement. (NOTE: A business requirement can be indirect, like the impact having 1 million of these running every day at full load).
As an example, we have an update statement taking 2 seconds. It feels slow to me. It's processing almost a million records. Part of a bigger process that is taking too long by 30 minutes. Not in a loop. I think I'll pass. [There is an 8 hour solution that would remove this. I logged a note, not in this pass. Making that update twice as fast is meaningless in this context]