Bob,
Regarding slow speeds Shays, Climaxes, Heislers were all geared locos intended for lumber, mining, and other uses on minimal cost trackage. They weren't designed for high speeds! Basically minimal effort made to grading, roadbed, curves, bridging, and the like. In a lot of cases the basic topography was pretty extreme for locomotive use. Speeds had to be low, and in a lot of cases rod locos couldn't operate in the conditions.
On Narrow Gauge "common carrier" roads the slow speeds were primarily a function of economics. The idea was to keep fuel, lubricant, wear and tear (loco maintenance) and other consumables costs down. The rail was kept as light as possible initially to keep initial costs down. Also, light rail can't support speed. Later most successful roads replaced with heavier rail as tonnage and passenger loads dictated. Management also had to deliver positive returns to the investors.
Also look at the very premise of Narrow Gauge:
These were roads built in regions that economically couldn't support Standard Gauge roads. Later, all the Narrow Gauge roads that had sparked adequate economic development were Standard Gauged.
Rod locos were essentially scaled down standard gauge equipment and were fully capable of high speeds (if the basic purpose of the loco wasn't specialized for slow speed, high tonnage hauling).
The basic 4-4-0 American (1870's-1920's) was capable of 60 MPH but rarely ran that fast as most Narrow Gauge roads trackage simply wasn't designed or built to support such speeds! Also, management generally couldn't condone the fuel and consumable costs due to most roads traffic loads. Two of my roads of interest, the NPC and SPC (in California) had a few areas that could support moderately high speeds up to about 30-40mph (curvature, grade, commuter service, local express service). Generally they averaged 24mph due to station stops for passenger pick-up and refueling). Freight service was a slow 12mph average as dictated by economical reasons.
A book everyone interested in Narrow Gauge should have:
"American Narrow Gauge Railroads" by Hilton. It's basically one of the bibles on the subject.
CraigH