reducing MIPS in detecting tones
Assuming that the tones are sufficiently sinusoidal, here's a possible scheme, with a bandpass filter being controlled by a frequency estimator:



If the tunable bandpass filter's output is at about the same power as the input signal, then the input consists mainly of a sinusoidal tone. It is perfectly fine if the frequency estimator is a simple one that only gives a meaningful output for the tone to be detected.