Assuming the ISR 4K supports it, and you have sufficient flash, in theory, decompressing the IOS, saving that on flash, and booting it should save a bit of time.
Further, also assuming an older IOS version or a less feature set rich version (now a days less likely as Cisco often provides universal versions) can be used, that's also physically smaller, should also, in theory, save a bit of time booting it up.
One other possibility, if your ISR has some form of external insertable flash memory, from which it can boot from, and you if you can obtain various performance levels for that flash type, using such, again in theory, might save a bit of boot time.
Although any and/or all of the prior may save some boot time, I'm unsure there would be a huge savings, first because IOS (now a days) is often larger than ever (and continues to grow) and possibly Cisco hasn't designed their devices with fast booting in mind (something which you might suggest to them).
