Lets remember Broad is now 30, nearing the end of most bowlers careers, he's played alot of test cricket in the last 5/6 years after his string of injuries, Also I think with the weight on his shoulders of the legend that is Jimmy injured, I think he's taken on a rather different role within the bowlers as the mule that just keeps bowling outside off and troubling batsmans patience rather than attacking.. and I'm comparing this to the 2013 ashes series where he too 6 for 20 in 45 balls! bowling at 90+mph... I believe he still has that in him but doesn't need to spend his time doing such as there are other wicket takers in the team.