The fastest luau base64 library in the west. This has been benchmarked within Roblox Luau, results may vary

b64.lua

local SEQ = {
	[0] = "A", "B", "C", "D", "E", "F", "G", "H",
	"I", "J", "K", "L", "M", "N", "O", "P",
	"Q", "R", "S", "T", "U", "V", "W", "X",
	"Y", "Z", "a", "b", "c", "d", "e", "f",
	"g", "h", "i", "j", "k", "l", "m", "n",
	"o", "p", "q", "r", "s", "t", "u", "v",
	"w", "x", "y", "z", "0", "1", "2", "3",
	"4", "5", "6", "7", "8", "9", "+", "/",
}

local STRING_FAST = {}
local INDEX = {[61] = 0, [65] = 0}

for key, val in ipairs(SEQ) do
	-- memoization
	INDEX[string.byte(val)] = key
end

-- string.char has a MASSIVE overhead, its faster to precompute
-- the values for performance
for i = 0, 255 do
	local c = string.char(i)
	STRING_FAST[i] = c
end

local b64 = {}

function b64.encode(str)
	local len = string.len(str)
	local output = table.create(math.ceil(len/4)*4)
	local index = 1

	for i = 1, len, 3 do
		local b0, b1, b2 = string.byte(str, i, i + 2)
		local b = bit32.lshift(b0, 16) + bit32.lshift(b1 or 0, 8) + (b2 or 0)

		output[index] = SEQ[bit32.extract(b, 18, 6)]
		output[index + 1] = SEQ[bit32.extract(b, 12, 6)]
		output[index + 2] = b1 and SEQ[bit32.extract(b, 6, 6)] or "="
		output[index + 3] = b2 and SEQ[bit32.band(b, 63)] or "="

		index += 4
	end

	return table.concat(output)
end

function b64.decode(hash)
	-- given a 24 bit word (4 6-bit letters), decode 3 bytes from it
	local len = string.len(hash)
	local output = table.create(len * 0.75)
	
	local index = 1
	for i = 1, len, 4 do
		local c0, c1, c2, c3 = string.byte(hash, i, i + 3)

		local b = 
			bit32.lshift(INDEX[c0], 18)
			+ bit32.lshift(INDEX[c1], 12)
			+ bit32.lshift(INDEX[c2], 6)
			+ (INDEX[c3])
		

		output[index] = STRING_FAST[bit32.extract(b, 16, 8)]
		output[index + 1] = c2 ~= "=" and STRING_FAST[bit32.extract(b, 8, 8)] or "="
		output[index + 2] = c3 ~= "=" and STRING_FAST[bit32.band(b, 0xFF)] or "="
		index += 3
	end

	return table.concat(output)
end

return b64

bench.md

The following benchmark was compared against the Base64 library available here using Boatbomber's plugin

Data was tested with 100,000 char strings

	decode	encode	AGF decode	AGF encode
10th %	2.751ms	4.755ms	5.185ms	7.554ms
50th %	2.997ms	5.123ms	7.474ms	9.015ms
90th %	3.482ms	5.864ms	8.847ms	10.030ms
Avg	8.934ms	11.329ms	27.008ms	20.659ms
Min	2.519ms	4.468ms	4.417ms	6.623ms
Max	15.348ms	18.191ms	49.599ms	34.694ms
Average bytes per second	111,931,945.377	88,269,044.046	37,026,066.350	48,405,053.487

going to attach some benchmark results, hold on

going to attach some benchmark results, hold on

You should benchmark it against https://github.com/Reselim/Base64 too