Rar 5 has proven specifically for unpacking game ISO's to be extremely fast compared to 7z native. The 7z.exe CLI took is wonderful and powerful to wrap around. It is also deeply embedded in the emulator front end so it may not just be replaced.
Here is an example of the performance change:
Legend of Zelda, The - The Wind Waker (USA) (GamdCube) size 1.459.978.240 MB
7z 00:04:40 1.182.305.393
rar 4.XX 00:01:14 1.163.525.401
rar 5.00 00:01:49 1.149.296.570
rar 4.XX 00:00:13
rar 5.00 00:00:09