Generally, the lowest speed is more than enough. Your pump is also oversized for what is needed. Hot water naturally wants to rise, so you don't need much to get it into motion, and the falling water on the return side helps draw it along. Using a too large pump can literally wear out the insides of the pipes. I think you said you had an 1/8Hp pump...that's probably 3-4x larger than needed, although its lowest speed may be moderately close. Note, a pump used on potable water must be either bronze or stainless steel or the thing will rust out from inside because of the dissolved gases in the potable water supply.
The system I have has an adjustable aquastat...I have mine set so that the furthest location gets to just over 100-degrees. That puts full hot water close by essentially everything prior to it, and leaves warm water always at that vanity sink. Some of the systems use 105-degree settings. More doesn't really buy you much and ends up costing more. It does this easily with a 1/28Hp motor. You're not trying to power a fire hydrant, and even after the timer turns it on, even if it takes a couple of minutes to get the pipes warmed up, that's usually irrelevant, and it should stay that way until turned off. If I'm awake when mine first turns on, it might run for maybe 90-seconds to maybe 2-minutes if it was quite cold overnight, about the same time as it takes while running the shower before it gets warm if I'm up before the timer kicks in. The copper tubing institute calls for NGT 5fps of flow with hot water. I don't know the number for CPVC, but it's probably similar. Too large of a pump, and you can easily exceed that water velocity, leading to noises, and erosion, not counting the extra power to run it.
Does your system have a check valve in it? Generally, that's required or you may be sucking water out of both the top and bottom of your WH when using water. The water will flow in the path of least resistance. Also, it can allow water to circulate even if the pump is off in some circumstances.