If you wish to improve the boot time, avoid using initramfs.
Rationale is that intramfs requires copying the data twice.
First the data needs to be loaded either by the boot loader or as part of your kernel image, and next the data is copied from the initramfs image to the buffer cache in the kernel.
Another disadvantage of initramfs is that the initramfs contains complete files. So if you have e.g. glibc in your initramfs it will be put into the buffer cache completely. However, only part of it will be needed as no-one uses all functions in glibc. If only part of a library or executable is used, the time to read, uncompress and copy the other parts is wasted.
Better just use the files directly from disk or flash. If your buffer cache is big enough they will stay in the buffer cache after the initial load anyway (and if your buffer cache is small you probably cannot afford to use initramfs at all).