I guess it’s highly dependent on the type of the tube. The documentation [1] for the 6N3P (the one Mike uses) talks about input resistance at 100 MHz, so the tube must handle at least as much.
Keep in mind that's in relation to amplifying analogue frequencies around 100MHz. It doesn't necessarily mean you can make digital logic that runs at 100MHz
[1]: https://www.istok2.com/data/575/