I'm suspecting it's prop pitch management but i can't be sure until you tell me what kind of RPM you were using during different stages of the fight.
As a rough pointer:
Go to a coarser pitch (lower RPM) during dives and you'll pick up speed much easier, then start adjusting towards fine pitch during the climb back up.
I usually adjust pitch to maintain about 2000-2100 RPM when diving and it picks up speed really fast. On the other hand, i only use finer pitch in a dive when i actually want to prevent it from picking up speed.
Diving with your RPM at 2500 is like driving downhill on 3rd gear, it will go faster if you switch to 5th gear (going to coarser pitch)
The useful powerband is of course pretty limited (2000-2500 RPM) and getting used to how much RPM you need for a given attitude and airspeed range takes some time getting used to, but it's definitely doable. You just don't get the performance for free anymore like we could in IL2:1946.
I don't fly online yet until some more bugs get squashed and we get some form of quality control for mods, but i have no problem almost rear-ending even the AI that many people constantly complain about its exaggerated performance, to the point that i've now set my cannon convergence to 100 meters and i would set it even lower if i could
The only thing that outruns me is AI Spit Mk.IIs and that's because it's the aircraft with the closest to real life performance in the sim, while most of the rest are a bit slower than they should be.
Heck, i can close in on AI Hurricanes and Spit Mk.Is with a 110 in a climbing contest if am careful. You just need to familiarize yourself with how prop pitch works and flying a few free flight and quick combat missions in the QMB is the quickest and easiest way.
I just pretend i'm driving a manual transmission car and act accordingly:
climb/dive = uphill/downhill driving
high RPM = low gear
low RPM = high gear
Hope it helps