What mechanisms limit the top speed of high altitude winds on Venus? According to http://www.universetoday.com/36816/winds-on-venus/, the high altitude winds on Venus travel at about 100 m/s. 

I don't think frictional resistance is really what's limiting the speed of the winds. Here's a possible cause of resistance to the winds moving even faster. 


Venus has a single convection cell per hemisphere. 
The atmosphere cools by adiabatic expansion as it rises at the equator, 


then because it's colder than the low altitude air, it emits radiation at a lower rate than the low altitude air 
so it heat's by absorbing radiation from the low altitude air faster than it cools by emitting radiation 
so the temperature drops less rapidly with altitude than that from adiabatic expansion. 

As the air at the equator rises, it cools 


but then it's colder than the rest of the air at that altitude 
so it resists being accelerated upward by the high pressure of the air under it. 

Also because of that resistance, it rises slowly enough that it heats up from absorption fast enough to not drop further in temperature as it goes higher preventing it from exerting a stronger force of resistance to the flow of the convection cells than the driving force for the convection.
I found some recent papers which, if nothing else, discuss apparent mechanisms.
Quoting from Schubert, 2016,


  The large-scale circulation of the upper atmosphere from ~90 to ~200
  km altitude (upper mesosphere and thermosphere1 ) is a combination of
  two distinct flow patterns: (1) a relatively stable
  subsolar-to-antisolar (SS-AS) circulation cell driven by solar (EUV-
  UV) and IR heating, and (2) a highly variable retrograde superrotating
  zonal (RSZ) flow, in part a continuation of the lower-atmosphere RSZ
  flow discussed above


Then, ESA says, in part,


  In 2006, average cloud-top wind speeds between latitudes 50° on either
  side of the equator were clocked at roughly 300 km/h. However,
  detailed cloud tracking studies revealed that these already remarkably
  rapid winds are becoming even faster, increasing to 400 km/h over the
  course of the mission. The reason for this dramatic increase is
  unknown.
  [emphasis mine]


Alternatively,  these guys think they have a working model:


  The atmospheric circulation in Venus is well known to exhibit strong
  super-rotation. However, the atmospheric mechanisms responsible for
  the formation of this super-rotation are still not fully understood.
  In this work, we developed a new Venus general circulation model to
  study the most likely mechanisms driving the atmosphere to the current
  observed circulation. Our model includes a new radiative transfer,
  convection and suitably adapted boundary layer schemes and a dynamical
  core that takes into account the dependence of the heat capacity at
  constant pressure with temperature. The new Venus model is able to
  simulate a super-rotation phenomenon in the cloud region
  quantitatively similar to the one observed. The mechanisms maintaining
  the strong winds in the cloud region were found in the model results
  to be a combination of zonal mean circulation, thermal tides and
  transient waves. In this process, the semi-diurnal tide excited in the
  upper clouds has a key contribution in transporting axial angular
  momentum mainly from the upper atmosphere towards the cloud region.
  The magnitude of the super-rotation in the cloud region is sensitive
  to various radiative parameters such as the amount of solar radiative
  energy absorbed by the surface, which controls the static stability
  near the surface. In this work, we also discuss the main difficulties
  in representing the flow below the cloud base in Venus atmospheric
  models


.