using await Task.Delay in a for kills performance

Task.Delay is lightweight but not accurate. Since the loop without delay completes much faster, it sounds like your thread is going idle and using an OS sleep to wait for the timer to elapse. The timer is checked according to the OS thread scheduling quantum (in the same interrupt handler which performs thread pre-emption), which is 16ms by default.

You can reduce the quantum with timeBeginPeriod, but a better (more power efficient) approach if you need rate limiting rather than exact timing is to keep track of elapsed time (the Stopwatch class is good for this) and number of calls made, and only delay when calls made have caught up to elapsed time. The overall effect is that your thread will wake up ~60 times per second, and start a few work items each time it does. If your CPU becomes busy with something else, you'll start extra work items when you get control back -- although it's also pretty straightforward to cap the number of items started at once, if that's what you need.

public async Task Generate(int numberOfCallsPerSecond) 
{
    var elapsed = Stopwatch.StartNew();
    var delay = TimeSpan.FromMiliseconds(1000/numberOfCallsPerSecond); // a call should happen every 1000 / numberOfCallsPerSecond miliseconds
    for (int i=0; i < numberOfcallsPerSecond; i++) 
    {
        Call();  // don't wait for result here
        int expectedI = elapsed.Elapsed.TotalSeconds * numberOfCallsPerSecond;
        if (i > expectedI) await Task.Delay(delay);
    }
}