That junction is, I think, called Scotch Corner, and is a terrible
bottleneck for Westbound traffic, partly because the Eastbound
carriageway has a longer green to allow right-turning, and partly
because the carriageway reduces from 3 lanes to 2 shortly after.

That the traffic there has improved may possibly be due to the
re-opening of the Hammersmith Flyover, which means less traffic is
diverting onto the A40, and that it is holiday season.