Less top speed, which is governed by many factors (consider that maintaining any constant speed in a vehicle requires some minimum power output to counter losses to heat/friction throughout the drivetrain and between the vehicle and oncoming air, not to mention the maximum safe operating speed of the drivetrain components themselves), but rather the potential for acceleration. If you look at the torque curve for some power plant, where the independent variable is oscillation speed and the dependent variable is output torque, you'll find that (for some fixed gear ratio between the power plant speed and the wheel speed) the maximum acceleration occurs at the point along that curve where torque is maximized. Electric motors have a much flatter torque curve with respect to rotor RPM than do combustion engines with respect to crankshaft RPM, but they still see a nontrivial drop in torque as rotor speed approaches its operating maximum. Transmissions exist to maximize the driver's access to torque: if you're traveling at 60mph on the highway with your engine spinning at 2K RPM, but your engine makes maximal torque at 4K RPM, you can shift downward to maintain your same vehicle speed while increasing the speed at which your engine turns over, thus giving you enhanced ability to bring your car up to speed before you merge with the lane to the left of you where everyone is driving much faster. A car that loses available torque at high power plant speeds and that lacks the ability to use such gearing to take advantage of optimal power plant torque will be unable to produce as much acceleration at high speeds as a car that has several gear ratios available.
Caveat: I'm just an automotive hobbyist, so if an automotive engineer rolls in and calls bullshit on anything I've said above, they're almost certainly correct.