They're going at 16.2km/h loaded and 21.6km/h empty without upgrades. Am I imagining things or did they become slower recently?

The readme says "Drones start with a speed of 22km/h, and a stack capacity of 1."

https://i.imgur.com/yEWvsqf.png

They get a speed penalty depending on what they are carrying.

function transport_drone:update_speed()
local speed = get_drone_speed(self.entity.force.index)
if self.riding_player then
speed = speed * 1.5
elseif self.fuel_amount then
speed = speed * 0.6
elseif self.held_item then
speed = speed * 0.75
end
self.entity.speed = speed
end

Oh, I see, thank you!