Most of the boot phases are required for operation and can't be skipped if you want a functional switch. Inflating the image, initializing first boot stage, decompressing and uploading microcode to the ASICs, Testing the ASICs afterwards, Initilizing IOS services, IOS has a different view of the system so test routines are run from higher level etc etc etc...
If short downtimes and fast recovery/convergence is crucial I can only think of "cutting" the stack in half and choose one of the L2 resp L3 redundancy/high availability options.