However, we can analyze the show's depiction and make some educated guesses:
* The speed is implied to be supersonic: The name "Sonic Rainboom" itself hints at this, and the show visually depicts the phenomenon as a massive sonic boom, creating a massive shockwave.
* It's far faster than a regular flight: Alicorn princesses like Celestia and Luna can create Sonic Rainbooms, which suggests a speed far beyond their normal flight capabilities.
* The speed depends on the pony's power: The strength of the Sonic Rainboom appears to be tied to the pony creating it. Powerful ponies like Celestia and Luna generate much larger and more powerful Sonic Rainbooms than ponies like Twilight Sparkle.
Conclusion:
While a precise number cannot be given, we can assume a Sonic Rainboom is at least supersonic and potentially even faster, depending on the pony generating it. It is undoubtedly a powerful display of speed and magical energy within the My Little Pony universe.