Help requested with FIR/IIR filters please

you're right about the FIR running fast because of the easy pipelining.

regarding linear phase, it will give you the most accurate wave shape which seems useful in your application. a consequence of linear phase is the undesirable delay you mention that does not happen with a (minimum phase) IIR filter. that said you could design a minimum phase FIR (less delay) but the wave shape would not be preserved as well as a linear phase FIR. you can also build a near-linear phase IIR which would have a similar delay to the FIR, but would preserve the wave shape.