Yeah, C2 or later models still have massive lag in 4:3 modes, which is a shame. A firmware update could fix it easily. It's only a problem if you use this one 1920x1440p resolution directly via Mister. If you use any other output resolution via Mister, or a scaler (like the RT4k or the Morph 4K which both have HDMI-in) then it uses the 16:9 native display resolution of your TV anyway (and frames the aspect ratio you want inside this resolution, which is mostly 4:3 or 1:1 for something like Game Boy). And probably direct video over HDMI. Which is the best combi you could hope for. Believe me: Mister and Retrotink 4K via OLED is a dream.
Mister Laggy measurements via my Sony Trinitron CRT TV (60Hz)
top = 0,750ms
middle = 6,88ms
bottom = 13,74ms
The bottom value isn't 16,7ms probably due to the missing lines/not fully visible overscan area. Which is probably normal for the most (if not all) consumer TVs.
btw: My results with VRR on and off for 1080p@55Hz, LG OLED C9 (which is important for arcade games with weird refresh rates e.g. R-Type). As you can see, VRR on helps here a lot.
bottom VRR on+vsync adjust_2 enabled =18,6ms
bottom VRR off+vsync adjust_2 enabled =30,7ms