The Short Answer: VOLTAGE
The max speed of any standard non-electric bike is determined by many factors: weight of the rider, flatness of the terrain, type of terrain (paved, gravel, dirt, etc.), position of rider (upright vs. crouched), width of tires, type of tire grip, size of the wheels, number of gears, and the strength of the rider. Won’t get into these factors since they are more important to standard bikes, whereas the top speed of an electric bike does account for variations of these factors, the main two components for max speed are the size of the motor and battery, specifically the voltage output of the battery and the voltage allowance in the motor. The battery voltage should match that of the motor, so we don’t need to discuss both.
(the following primarily applies to hub motors, since the wheel rotation speed limits what can be done with gearing etc.) In simplest form, the higher the voltage, the faster an eBike will go. The increase in voltage vs. max speed is not a complete linear relationship unless you are in a vacuum with no surface friction. So for real world, practical sake, there is a gradual decrease in the rate of max speed as voltage increases due to wind resistance and surface friction. This car be seen with cars the same way. The top speed of a car will not double if you simply double the horsepower. In fact, it takes exponentially more power to continuously increase the top speed because wind resistance increases so quickly the faster an object moves through air. Of course there is the factor of transmission, or gearing, when it comes to top speed. But for most electric bikes, the gearing is setup to allow for decent low-end and uphill performance and decent top speed. Gearing an eBike for the highest speed possible will cause a reduction in low-end performance, so most eBikes are setup for a happy medium.
(the following is assumed with both motors at 48 Volts) A 500 watt ebike motor could hit 28 mph, but that also depends on rider weight, rider strength, and how long the rider wants to stay up around 28 mph. (this also depends upon battery amps allowed to get to the motor. Usually 500 watt motors will have an 18 to 22 amp controller with them, but I have seen lower, and higher amp controllers. Whereas A 750 watt motor could more readily hit 28 mph with a heavier rider, who might be weaker than the rider on a 500 watt motor. Why ? Because typically a 500 watt motor is a nominal rating, allowing for perhaps a peak of 700 watts instantaneously, and the 750 watt motor as nominally rated, would have perhaps a peak of a 1000 watts. And the 750 watt motor, could be more likely paired with a 22 to 28 amp controller, so more current is going to flow to the motor. My point of mentioning this, is that a true 500 watt motor is typically paired with a certain size controller, and a true 750 watt motor is paired with a certain size controller that is usually of higher amps than would be paired with a 500 watt motor, but a lot of vendors are playing these games, where they are just pairing a higher amp controller and labeling that 'nominal 500 watt motor', a '750 watt motor', by multiplying Volts x Amps that flow from the controller to the motor. Or they might call a 350 watt motor, a 700 watt motor, doing the same thing. So unless you either ride the ebikes in a side by side comparison, and/or find out the exact specs of the controller, going just by the wattage of the motor, and rated volts of the on-board battery, really tells you nothing.
To take this one step further, you can take any motor, and up both the amps of the controller, and increase the voltage to say 52 volts, and even get a bit more speed, and power. So really its best to think of wattage as the work that can possibly be done in terms of overcoming rider weight, and wind resistance, and rider weakness, and then voltage is determining the higher speed capability. Again this needs to take into account gear ratios of the ebike design, and a general goal of a 'happy medium' of not too little or too much power at the low end when starting out, to gain something better on the high end. You could build an ebike that had a top end speed of 35 mph, using just a 250 watt motor and 36 volts, but you would not at all, appreciate the monumental effort it would take you to get it going from a dead stop.
With mid drives, you'll see them able to achieve speeds of 28 mph, with a nominally rated 250 watt (sometimes a peak of 350 or even 500 watts), 36 volt motor, BUT the big difference is, they are not limited by the rear wheel speed like a hub motor is, and the internal motor on the mid drive is spinning at very high RPM's (often 4000), and thus they are able to optimize the torque and power output by utilizing more gear ratios INTERNAL to the motor, and then leverage that further through the front and/or rear cassette gears on the ebike.
You asked a simple question, but the answer is far from a simple or direct answer (thanks in large part to the marketing games that are played by ebike OEMs, and the lack of transparency on all the important specs - not just showing wattage and voltage or even torque. )
