FWIW, SBCL is pretty good at optimizing away dynamic type checks if you help it out.

Here are some examples under:

    (declaim (optimize (speed 2)))

First example is a generic multiplication. x and y could be _any_ type at all.

    (defun fn (x y) (* x y))

If we disassemble this function, we get the following:

    ; disassembly for FN
    ; Size: 34 bytes. Origin: #x1001868692                        ; FN
    ; 92:       488975F8         MOV [RBP-8], RSI
    ; 96:       4C8945F0         MOV [RBP-16], R8
    ; 9A:       498BD0           MOV RDX, R8
    ; 9D:       488BFE           MOV RDI, RSI
    ; A0:       FF142540061050   CALL [#x50100640]                ; SB-VM::GENERIC-*
    ; A7:       4C8B45F0         MOV R8, [RBP-16]
    ; AB:       488B75F8         MOV RSI, [RBP-8]
    ; AF:       C9               LEAVE
    ; B0:       F8               CLC
    ; B1:       C3               RET
    ; B2:       CC0F             INT3 15                          ; Invalid argument count trap

Note that it calls `GENERIC-*` which probably checks a lot of things and has a decent overhead.

Now, if we tell it that x and y are bytes, it's going to give us much simpler code.

    (declaim (ftype (function ((unsigned-byte 8) (unsigned-byte 8)) (unsigned-byte 16)) fn-t))
    (defun fn-t (x y) (* x y))

The resulting code uses the imul instruction.

    ; disassembly for FN-T
    ; Size: 15 bytes. Origin: #x1001868726                        ; FN-T
    ; 26:       498BD0           MOV RDX, R8
    ; 29:       48D1FA           SAR RDX, 1
    ; 2C:       480FAFD7         IMUL RDX, RDI
    ; 30:       C9               LEAVE
    ; 31:       F8               CLC
    ; 32:       C3               RET
    ; 33:       CC0F             INT3 15                          ; Invalid argument count trap*