if you send all that video data through the ddr4 bus twice (once writing, once reading) you're using about 20–25 percent of the machine's memory bandwidth, but the linux tty subsystem actually makes several copies and runs many instructions per byte [citation needed] so you can't get anywhere close to that in practice. especially not if you're spending a lot of time repacking bits into six-bit bytes in order to feign compatibility with the vt340, a misguided engineering dead end from 37 years ago that only supported 16 colors and 800×480 anyway
other protocols like x11 and xpra work considerably better, but x11 video reproduction on a local terminal normally uses xshm or its moral equivalent with xvshmputimage or gl. if you're on a local terminal, you might as well use x11; the only benefit to putting the pixels in the same bytestream as text is if the your terminal is connected to your drawing application over a bytestream such as an ssh connection, a tcp connection, an spi connection, or a uart
i think we could do better by designing a bytestream protocol that minimizes copies and pixel format conversions and is therefore within epsilon of the speed of xshm, but sixel isn't it, and neither is x11 without xshm. a bidirectional bytestream protocol could include flow control to also avoid transmitting pixels that won't be visible and increasing latency due to bufferbloat
as for emoji, they don't improve information density; there are only 2666 emoji in unicode 10.0, so at a maximum they convey only 13 bits of information, and they normally occupy roughly the space of two letters like 'n', so you have slightly fewer bits per pixel. people use emoji because they are cute and colorful, not because they are ithkuil. they're not
some technologies, like the unix shell, smalltalk, and tcp/ip, make hard things easy and easy things possible. others, like retrocomputing, code golf, and malbolge, make easy things hard and hard things impossible. sixel makes easy things hard and hard things impossible, like the rest of retrocomputing.
that's a fun way to spend my time when i choose to (e.g., in https://asciinema.org/a/390271 i did real-time 3d graphics in a unicode terminal emulator with braille) but it's not how i want my primary user interface to my computer to work