Calculation in LTspice "never" finishes
When I get those errors, it is usually because the SPICE model from the manufacturer is wrong or broken, or that your SPICE settings are wrong (open the LTspice's Control Panel and click Reset on the SPICE tab).

I tried implementing the circuit from your application note in LTspice and it worked (it is here), but:


I used the LT1720 since it was already in the LTspice libraries has a 4.5 ns propagation delay and the MAX9010 has 5 ns;
The XOR gate is set to have a 4.8 ns propagation delay (from the datasheet).
The clock source is 15 MHz.




This gives me:



As you can see, the pink trace is roughly twice the frequency of the green one (the input signal), working as expected.

I tried using the MAX9010 SPICE model, but had to edit it to put the pins in proper order, and the output looked wrong.