A few extra comparisons and output for each byte shouldn't be that much slower; fortunately the function of this program is extremely well-defined, so we can calculate some estimates. Assuming a billion instructions per second, taking ~1.5s to hexdump ~1 million bytes means each byte is consuming ~1500 instructions to process. In reality the time above is probably on a faster CPU, so that number maybe 2-3x more. That is a shockingly high number just to split a byte into two nybbles (expected to be 1-3 instructions), convert the nybbles into ASCII (~3 instructions), and decide on the colour (let's be very generous and say ~100 instructions.)

The fact that the binary itself is >1MB is also rather surprising, especially given that the source (not familiar with Rust, but still understandable) seems quite small and straightforward.

(If it's spending a lot of time in Rust's format function you could also use a (or the same) lookup table to convert to hex/dec/oct.)