TT always work at same speed regardless of resolution, right?
Falcon is one that speed is affected by resolution and color numbers.
No, TT can slowdown too in higher res. and color modes. That's the reason why SW works faster from Fast RAM, which is not shared with video. Actually, TT and Falcon have approx. same video bandwith - first has wider bus, second has faster RAM chips.
And to add that in case of ST there is actually some 'engineering' reason: Video clock is synchronized with CPU clock, so in all video modes reading video from common RAM goes at same speed. In low res, 16 color mode 2x more data is needed than in med. res for 1 pixel. and because res. is half - reading speed is same. In high res there is 640px width too, as in med, but it is drawn at double speed - what makes possible ergonomic 70 Hz refresh rate. Syncing makes possible work without CPU slowdown, well - not 100%, in some rarer cases there is little wait.
But it is true that some overscan would be good in original design. And a lot of other things. Just remember that it's made by humans in limited time.