I2C speed is actually limited by the bus capacitance, although "cut-off frequency" isn't the usual term.You find respective calculations in the I2C specification by Philips/NXP. Due to the fact that the specification has fixed speed limits of 100/400 kHz, bus capacitance isn't a problem for usual applications.
SPI can be implemented with impedance matched transmission lines and doesn't suffer from cable capacitance in this case.
I'd say this high speed I2C is a very special mode.
You need
* a high speed master
* a high speed slave
* special wiring / PCB layout
Do you know a true "high speed master"? (I don't)
Do you know a true "high speed slave"? (I don't)
Do you know a usefull application where a high speed I2C has a benefit over SPI? (I don't)
For sure "capacitance" is one limit, but there are many other problems that prevent me from using a "high speed I2C" bus.
Don't get me wrong. I often use I2C. I use it for low data rate transfer because it's ease of use. But I don't find the high speed usefull.
In one system we used SPI over LVDS via a backplane in a 19" rack system. It's easy to transfer 5MBytes/s simultaneously in both directions. Impossible with I2C.
(For communication on one PCB one doesn't need LVDS.)