If you have access to an analog VGA output, the sync and video signals are a pretty good source for clean and fast rise and fall time signals.
I connected it to the sync signal of a VGA output and saw approx. 4.7 ns Risetime. I don't have much electronic experience, so I don't know if it really handles 100Mhz.
On the colour signals I was not getting clean signals. Resolution of the signal was 1440x900 which should give 106Mhz.
Bandwidth depends on transition time and not resolution. The last time I used a VGA output like this, even the sync signal edge times were faster than 300 MHz (*) so your 4.7 nanosecond rise time implies a bandwidth of 75 MHz (0.35 / 4.7e-9). I am not sure how much faster than 300 MHz they are because that is the bandwidth of the fastest oscilloscope I have tried this with which did indeed report about 1.15 nanoseconds.
The color signals are more difficult to use but green should have the sync signals embedded in it. If your oscilloscope supports TV triggering, then it should be able to use the green signal just like a composite video signal. All of my Tektronix oscilloscopes which support TV triggering have worked correctly with the green VGA signal as the trigger source which I thought was pretty amazing given that they were intended for NTSC and PAL. My ancient Tektronix 2440 DSO with the TV trigger option had no problems counting as many high definition scan lines as I could generate which was 1200 and more.
(*) VGA pixel clocks at high resolution require bandwidths of 300 MHz and higher but the performance implied by that bandwidth applies at any resolution. In the past, VGA card bandwidth was part of the specifications because even at lower resolutions, it has an impact on display clarity.