I was trying to use TL431 regulators as much as possible on this design so they're used for the regulator, the integrator and the voltage amplifier. The only thing that I didn't use one for is the buffer. Unfortunately, that set the frequency response as the GBW is only around 1 to 2 MHz so the amplifier stage limits us. What happens if we modify it?

I snuck a sneaky option into the amplifier design. You'll notice a 0R and a not fit resistor. What's the point of that? Well if you remove R111 and replace R112 with an 0R then you get something that looks a bit like a classic common emitter amplifier.

 Change the values of the surrounding resistors (R118=20R, R117=1k5, R107=9k1k) and bodge an mmbt3904 in place of the TL431 and you've got a nice high speed amplifier with twice the gain of the original. You may need to tweak these values if you make one due to part to part variability. I'm probably pushing it a bit hard asking for 40x (~32dB) gain.

Before:

After:

Now the 100kHz triangle is sharper. It's also got a lot more noise as the amplifier isn't filtering it out any more. I know that the circuit being tested is poorly laid out so most of the noise is probably genuine.