The fastest bus line in Vienna is the 55A driving from Speising to the Lainzer Tiergarten (a large park) on the Hermesstraße. It completes it´s 2km long route with six stops along the way in 5 minutes. That makes it´s average speed 24.0km/h, faster than Vienna´s fastest tram line, the 60, with 19.4km/h and even many Subway sections. Here is the 55A on the map.
It basically is a straight line with no turns. It is a bit unfair to compare the 55A´s speed to other bus lines. The 55A drives on a street that does not see heavy traffic, the 6km long 62A which has similar straight route only manages a 17.7km/h speed, it runs on the traffic jam prone Breitenfurter Straße. But lines that run on similar light traffic roads as the 55A loose speed due to their snaking routes. The 60A has an average speed of 18.8km/h, the or the 16A manages almost 16km/h.
The turns really slow the bus lines down. Often the 61A will wait a whole traffic light cycle to be able to turn from the Carlbergergasse south onto Brunner Straße. The 60A will also often wait opposite from it on the Carlbergergasse waiting to turn North onto the Brunner Straße. The stops of both of these lines are on the other end side of Brunner Straße, meaning if you want to change lines there you might wave good bye to your connection whiles waiting for the pedestrian signal to turn green.
Routing a bus line straight along the Carlbergergasse and straight on the Brunner Straße and on similar intersections would speed up the network. Scrapping detours up and down hills would increase the speed of the network by reducing turns. It would be nearly impossible to reach the 24km/h speed of the 55A, but increasing the average network speed in the south of Vienna by +2km/h to +3km/h would go along way.