Simplified:
AGA has 3 different (fixed) pixel-clocks:
LowRES,HighRES and SuperHighRES, each doubling to the one before.
SuperHighRES in 8Bit needs such a bandwidth that it uses up allmost all clock-cycles that would normally allow the CPU to access the ChipRAM. Thats why it feels so slow.
HighRES in 8Bit only uses the clock-cycles that are assigned to the chipset anyways.
All (AGA) screenmodes are based one of these, increase the pixel-count and you have to reduce the refreshrate and vice-versa.
One could in theory create a 1280x1024 screenmode based on LowRES, problem is that it would refresh only at about 3Hz. Same mode in HighRES 6Hz, SuperHighRES 12Hz ...
Actually one can get slightly better refrsh-rates by useing overscan.
