xtal start-up time
Start-up time depends on gm, especially on gm-opt, which is the optimum gm for a crystal oscillator to have the fastest start-up time.
Negative resistance -R(c) reaches max. when the gm reaches optimum.
The start-up time constant (r) depends inversely with R(c), so the larger R(c), the smaller time constant. That's why you see you increase gm, you decrease start-up time. Actually, there are another factors that controls your start-up as well. It is the DC biasing point where X1=X2 since when you starts up the circuit, it takes some time to reach X1=X2 and then both signals oscillates from small to large amplitude, so the large DC bias current, the faster X1=X2 and so it may also reduce the time to oscillate too.
You can simulate all the way from very small gm to very large gm to see the start-up time and you can find a peak for smallest start-up. Make sure you check it is gm, instead of biasing current. From this point, it is also meant that your design is not come to a optimum point where you still have margin to improve or trade-off between start-up time and power consumption.