Slight issue with the HP calculation; Horsepower is just a different unit for measuring power, and yes, it is equal to ~745 watts. That said, measuring power going into a motor will not tell you how much power you get at the shaft. The problem is inefficiency. No system will take in 100 units of power and give out 100 units of power, its not possible with humanities present understanding of thermodynamics. Universal motors, the kind powering your saw, generally have an efficiency rating of 50-70 percent, probably on the lower end in this particular case, so 1hp is a much more accurate rating for what youre getting at the shaft.
Problem the second is again with how horsepower is calculated. The equation is HP=(RPM*T)/5252. Where RPM is obvious and T is equal to torque. RPM is how fast something spins, torque is how hard it is to stop, and as you can see in the equation, you can raise one, lower the other and get the same number at the end.
Now, to illustrate this, your saw is rated to consume 15 amps, which should theoretically work out to 1.5hp with the inefficiency factored in, so far so good. Then you get to the blade rpm, 5000rpm. Plug that equation and you get 1.5=(5000*T)/5252, which means that torque is equal to 1.5756ftlb, assuming Wolfram Alpha did the math right because im lazy. For the same of comparison, lets use the more common 3600rpm that most induction motor saws spin their blades at. So, 1.5=(3600*T)/5252, T=2.188.
So, Tl:Dr version of all this is that yes, torque is the issue. Your saw doesnt have the torque needed to power through the cuts, which is why your saw is bogging down on heavier cuts
Also, Chickn already went through the math but switching to 220v wont make more power magically appear. The formula for wattage is W=A*V. The wattage consumed by your motor wont change, so either way your equation will look like 1500 (or whatever) = 120*A or 1500=240*A. Only thing that changes is the amperage, which could be a plus as lower amperage is healthier on the wiring,but that wont give you any more power