How quickly can they detect changes in magnetic fields? Is it slow like a cds cell, or fast like a photo-transistor? Bassically, if I were to quickly move the sensor through several different magnetic fields would it be able to handle it or would there be a lag time?
It depends on which specific device you had in mind, but typically they have response times in the single-digit microsecond range. [uarl=http://www.gmw.com/electric_current/Sentron/CSA-1.html]This one, for example has a <6 µs response time, according to the data sheet.
The TLE4906H switching hall IC is quoted with a delay time of 13 microseconds typ, and the linear-output IC TLE4990 with a -3 dB frequency of 1.6 kHz typ. (just pulling two types at random from the Infineon catalog) Looks to me more like (slow) phototransistor than like CdS range.
I once used a hall effect sensor to sense a magnet on a spinning arm. I don’t recall the exact angular speed, but it was something like 2000 RPM. Anyway, it worked.
2000 RPM is only 33 1/3 RPS. That’s easily within a <10 µs Hall sensor.