Typically the VFD will be better than 92% efficient. The speed can be set to virtually any speed in its range, but most of the controls have specific steps. This is not a 2 speed pump using different windings.

The issue is that with centrifugal pumps, the power goes up with the square of the speed while the flow goes up in proportion to the speed. The flow will reduce some if the discharge pressure goes up with the higher flow, but it is small compared to the parabolic increase in the pump.

Not an issue at 1 HP,, at 300 hp yes... Most of the issues discussed here were fixed in the late 80's. Same with electrical noise if good grounding practices are followed not an issue.

I am correcting an issue from the install of my Pentair variable speed pump. The pool guy removed the timer that used to run the single speed pump and the salt water chlorinator system. He then hooked both up to constant power. The SWC is only supposed to be operated when water is flowing through the cell...not 24 hours a day. He asked his pool guy "group" at a meeting and they all agreed that it shouldn't harm the SWC despite the fact the install manual says it about 6 different times. I got with the SWC manufacturer tech support and they said no way, it will damage the cell. The cells are not cheap.

If you all have a SWC and they change to a variable speed pump you might want to make sure the SWC is properly installed after the change.