Sure, but at what altitude is that speed penalty? The Bf109K was able to clean things up, but it's biggest boost was in the improvement engine and boost configuration.
Looking at Fw 190A-8 (has HMGs) and A-6 (has LMGs), seems the speed penalty was at all atitudes - eg. 550 vs. 560 at SL, 645 vs. 655 at 6.5 km.
109K-4 shown that 20+ km/h can be earned vs. 109G-10 (the two have had same engines, DB 605D) via cleaning up the HMG installation + undercarriage being fully retractable & covered. The 109s using big-superchager versions of the DB 605 (like 605AS or 605D) again show a performance increase of another ~20 km/h vs. usual DB 605A above 7 km (= right where needed in 1943-44 in West Europe).
Obviously, all of the much improved DB 605s were too late to matter, while a 2-stage supercharged DB 628 was not proceeded with, and another 2-stage S/Ced engine, the DB 605L, never entered service being too late.
Not really as I did say 'available', as the C3 fuel was not really available by Summer 1944 and prior was mostly saved for the Fw190, which required it for it's engines to work.
C3 was certainly available by early 1942 until the end of the war. Granted, not in some amazing quantity, at least until the Allied managed to bomb the fuel infrastructure.
Do you have some info about what you're talking about? All I'm seeing on a quick search is the consequences of high altitude and lift penalties due to how thing the atmosphere is. Speed rises though the further up you go due to reduced drag so long as the engine gets enough atmosphere to keep up combustion.
Sharp rise of compressibility-related drag was a reason why people went to swept wings, since that was once way to delay onset of compressibility. Another way was to make wing as thin as possible - this is where Spitfire excelled, for example, as well as Tempest/Sea Fury and Spiteful. Or to use wing profile where thickes part of wing is at 40-50% of the chord (P-51, Tempest, Sea Fury, Spiteful).
Note sharp rise of wing's drag coefficient
beyond 0.65 Mach. 0.65 Mach = around 440 mph at 30000 ft, but also 500 mph at SL (
graph). Contrary to that, the profile drag rise was not that pronounced.
List of aircraft suffering the low mach limit at high altitudes includes P-38 and Westland Welkin, for example.
Prop-driven aircraft in last 50 years use the phenomena to set speed limits at low-ish altitudes, since there the onset of compressibility is delayed by ~50 mph vs. flying at 30000 ft and engine power is double, if not triple of what the ww2 service aircraft had.