Darth Bane's speed feat

In the real world, Mr Olympia is 5'9" and weighs 280 pounds in the off-season (250 in the on-season). Extrapolating, it is not inconceivable for Bane to weigh 300 pounds.

Using these numbers (200 cm, 300 lbs) I used this online calculator to estimate Bane's surface area as 2.75 square meters.

The Wookiee lists a lightsaber as being "usually 145 cm" (1.45 m) long and a good estimate (backed up by an archived forum) guesses it is 4 cm (.04 m) wide. The surface area of a cross section of a lightsaber blade is thus .058 meters square.

Bane is successful at using is .058 square meter rectangle to defend his entire 2.75 square meter body surface area from rainfall. Dividing the two numbers shows that the lightsaber has to cover an area 62.5 times larger than itself every second. That's 62.5 places for the lightsaber to be every second.

One possible objection is that counting both the front and back of Darth Bane may overstate his speed. However, the narration of Bane's training exercise is very clear that he is not minimizing surface area, and that both his front and back is in danger at any given time:

"Tiny clouds of hissing steam formed as his blade picked off the descending drops while Bane twisted, twirled, and contorted his body to evade those few that managed to slip through his defenses."
-Darth Bane: Dynasty of Evil

If anything, the surface area of this analysis is understated (because I refuse to do calculus to track his exposure to the rain!)

Now, we have established that the lightsaber is covering an area 62.5 times greater than its own, every single second. Why is that significant? Well, the narration is clear that this is more than a light sprinkling of rain:

"As the first fat drops splattered onto the patio stones around him, Bane exploded into action. Abandoning the overpowering style of Djem So, he shifted to the quicker sequences of Soresu, his lightsaber tracing tight circles above his head in a series of movements designed to intercept enemy blaster bolts.
The wind rose to a howling gale, and the scattered drops quickly became a downpour. His body and mind united as one, he channeled the infinite power of the Force against the driving rain. Tiny clouds of hissing steam formed as his blade picked off the descending drops while Bane twisted, twirled, and contorted his body to evade those few that managed to slip through his defenses.
For the next ten minutes he battled the pelting storm, reveling in the power of the dark side. And then, as suddenly as it had begun, the tempest was gone, the dark cloud scurrying away on the breeze."
-Darth Bane: Dynasty of Evil

A government website lists the number of drops per second per square foot as 1,220. That means that the area covered by the lightsaber at any given moment experiences .058 * 1,220 = 70.75 raindrops per second. Flipping that, we get .014 seconds per raindrop.

If Bane were simply moving his lightsaber up and down then it would travel 2*2 meters (head to toe and back again) in .014 seconds. This results in a speed of 285 m/s. Sadly, Bane is spinning and jumping and greatly increasing the distance his lightsaber has to travel before it can return to any given point. Let's say that the twists, twirls, and contortions triple the distance that the lightsaber has to go. Then we get a speed of 857 m/s, which is very close to the muzzle velocity of an M16 rifle.

Bane is running bullet speed during his training period, which is a whopping 857m/s. Truly incredible.

Yep... at the very least it makes him faster than Grievous and Windu.