Developer yellows8 released HomeMenuHax which is the fastest (and probably easiest) method to boot homebrew so far. It works on firmwares from 9.0 to latest firmware 10.1. It can also be used to autoboot CFW if you have a 9.0-9.2 3DS/New 3DS.

3DS scene is full of hax, ninjhax, ironhax, youtubehax and now HomeMenuHax which is above all of them in my opinion. HomeMenuHax uses a vulnerability in theme loading. This way, it can take you to homebrew boot screen during the inital home menu loading just by holding the L trigger (it is customizable, even no button hold is possible). HomeMenuHax is fully capable of doing what ninjhax, ironhax and youtubehax can do. To install it, you need a vulnerability first, you can use the browserhax which still works in latest 10.1 firmware.

Due to the nature of HomeMenuHax, it needs a firmware with theme support, themes were introduced in 9.0.0-X on Old3DS, v8.1 on New3DS. and 9.1.0-XJ on Old3DS JPN. Unfortunately you can’t use themes with this hack as it messes with loading the hack.

As if fastest and easiest homebrew booting isn’t enough, developer delebile at GBATEMP ported Brahma to Ninjhax 2 (and all other haxes including HomeMenuHax). This means if you have a device between 9.0-9.2 firmwares, you can autoboot your favourite CFW with HomeMenuHax. No more need for cubic ninja cartridge, using browser for the hack or even MSET(DS profile exploit). This is by far the fastest CFW booting method available.

Here is a guide at GBATEMP to fully enjoy your auto boot CFW. Scroll down the page for Old 3DS instructions. Have fun, with the autoboot CFW, several emulators including Retroarch, Virtual Console game injecting and game hacking/modding, 3DS is a very customizable device nowadays.

Source for HomeMenuHax at GitHub, Brahma II at GitHub