IR2110 LTspice simulation weird behaviour
On startup, your bootstrap capacitor C3, C4 need be charged through D2 to VS then R1 to ground. If your R1 is too large, the charging time will be long. Assume R1 = 1k ohms, and C3 = 22uF, then

$$
\tau = C3 \times R1 \approx 22ms
$$

This causes about 50ms 10% ~ 90% rising time. Because the high side driver supply actually floats on your load, so as the capacitor voltage rising, the lower slope line reflect the voltage change on your load resistance. 

When you change the load resistor to 10 ohms, the time reduced to about 500us.