Hmmmm...I guess the question of the "easiest" way to increase speed hasn't been answered.
Let's take increasing the bow power and making the arrow lighter.
First the arrow: it would have to be both light in weight and strong in spine. Cane is the first thing that comes to mind as a material. I think it can be argued that rivercane has the best strength to weight ratio. How easy is it to find the right piece of cane and make an arrow from it? Very easy if it's available. If it doesn't grow nearby, then it's very difficult. If there isn't any cane, then you could always use a light weight wood species....but is that wood going to be strong in spine? Probably not. Overall, it's harder to make a strong lightweight arrow than a strong heavy arrow.
Next, the bow. If the limbs are wide and/or long, you can make a strong bow (with a long draw length) without worrying too much about breaking the wood. If there is wood available for this type of bow (long, wide logs with straight grain), then easy...if not, then difficult. In this case, what is easier to find? Long wide staves, or short narrow staves? I think the latter. Overall, I think it's harder to make a powerful bow than a lightweight bow (of the same draw length).
So, if dropping the arrow weight and increasing the bow strength is not the easiest, then what is? Good question. You can get a small increase in speed simply by switching to a lighter arrow, but then the accuracy suffers if that arrow is not matched to the bow. Most people today are not too concerned about spine anyway, so this seems the easiest....but to someone who's persnickety (like me) the notion of unmatched equipment is torture....and causes nightmares.
IMO, the easiest way to achieve greater speed is to increase your bow-making and arrow-making skills, and that includes properly matching the equipment.