Anything between the billions to trillions of times FTL, as a bare minimun the balls have fly across intergalactic distances due of Namek be in a separate quadrant, place it in another galaxy.
Beside if we want to mention underrated feats, you can also go with Namek Saga Goku be able to speedblitz Scouters as the ones of Burter and Jeice can't either track him his movement or he can rise his power level at max and then supressed it so fast that they can't read it.
The same Scouters who could actually elaborate and transmitt everything that happen on Earth to the planet where Vegeta and Nappa were present in a short period of imte, not just the video and audio but they could also read power levels.
In the Anime we also see in a filler where one of Frieza's planet there its a massive size scouter which could detect Goku and Frieza's power levels/Ki from Namek, making it even more consistent for Scouters to have MFTL+ processing/reaction speeds.
There its also Krillin, Gohan and Vegeta at Namek they could detect the Ginyu Force fly on their attack balls on Namek with their Ki Sensing,
and Ki Sensing its actually tied to the user's reaction speeds so it should scale to that at least.