If you haven't performed your LTHR test to determine your lactate threshold, you're just guessing as to what your zones are. You're only in Z5 if your LTHR is like 140, which is highly unlikely. Everyone is different, but for me, 145bpm is smack dab in the middle of Z2. My Z5 starts at 171bpm, and I don't even get into that zone most days.

Also, higher cadences bring down your HR. You will likely have a lower HR at 90rpm than you do at 70rpm. Mashing puts more load on everything, and your HR rises.