crbug.com/218244 introduced the concept of sticking to very low bitrates during the connection process (auth, assoc and IP config) by shill issuing a command, and then switching back (again from shill) once the connection succeeded. 
This is probably sub-optimal and only works today because all routers (irrespective of what config they are running a/b/g/n/ac) are required to support some basic rates (1Mbps, 6Mbps being among them) and so those rates can be used for the connection process. 
I wonder if routers will continue to support these basic rates for long.
This whole setup was introduced for ar9k and today's chips possibly dont have that issue. 
This has also caused us other problems in the past - the Intel wifi driver basically had to add a hack to say "if setting low initial rates results in no usable rates, ignore this command". 
Adding this bug as a "to fix some day".