That's a horrible circuit, but it would be improved with better sampling diodes. There are 0.25 pF diodes around, like SMS7621 or BAT15.
Classic samplers had blowby compensation to cancel signals leaking through the diode off capacitance.
I never has much luck with full-bridge samplers. Half-bridge worked for me, whit short impulses as the sampling gate. I did a theroretically 40 ps 1bit sampler/TDR which I never finished testing, but that's an easy way to go fast.