A big computer architecture debate of the 1980s was RISC vs CISC, arguing that building Reduced Instruction Set Computers was better than Complex Instruction Set Computers like the 8086. Most instruction sets since then are much easier to decode than x86, but x86 keeps going.
To learn about the Group Decode ROM in excessive detail, see my blog post https://www.righto.com/2023/05/8086-processor-group-decode-rom.html