I think they started well..

Given there are two main points to consider when in comparison with the bees knees ferrari team.. first being that the bmw engine isnt as fast in a straight line.. second is that the chasis doesnt handle as well meaning that the williams loses more speed in corners and wont exit as fast as the ferrari.

When they first started with willaims both these werent a huge problem. The willaims handled just as well if not better than the competition and straight line speeds were pretty much equal with other engine manufacturers.

I'm pretty sure that the Honda is faster than the Ferrari in a straight line but here you see the chasis letting it down as they too lose too much speed into corners and so their exit speed is far too slow causing them to fall behind.

So all up its likely that the williams chasis is letting BMW down more so than the BMW engine letting williams down. Unlike Mclaren who are having probles all over the place with engine speed reliability and chasis balance.

My 2 cents anyways