It's still too early to tell if that's going to end up being a problem results-wise, but from a scouting perspective, it does not seem good. I am all for Felix varying his fastball speeds a la Jarrod Washburn, but that's assuming he can still keep hitters further off-balance with a mid-80s change in addition to his high-80s slider and low-80s curve. But if his slider, changeup and 2-seamer all start to run together on the radar gun, then Felix just has their differing movements to make batters miss. That's probably still enough for Felix to get by because he does have incredible movement, but if he can re-couple that movement with changing speeds, it's going to make everything more effective.
— Lookout Landing: Felix's Disappearing Changeup LOL takes a look at Felix Hernandez's fastball increasing in speed, and what that means for his effectiveness. Complete with geometry!