Frequency is a time domain phenomenon, that does not exist instantaneously.

If you know a whole lot about the signal (what the waveform is, what the amplitude is, what the likely frequency range is, etc), then you can take a few quick samples of the signal, and fit those samples to your model of the waveform, to estimate the frequency. However, that's always an application-specific "tunable" process, not a "true" frequency measurement.

A good middle ground mostly-well-behaved signals is to measure the time between zero crossings -- typically in one particular direction (say, negative to positive). Keep the time of the last zero crossing, and the next time it happens, subtract that time from the current time. Invert, to get an estimate of frequency. You can keep multiple samples of zero crossings in a FIFO to measure across more cycles, and get more stability but slower response time to changes.