This is working data, so you'd keep it in a register if possible. Sadly that just happens not to be possible on the 6502 :)

Two bytes can be kept in the X and Y registers. Immediate load and add instructions only use two cycles.

    CLC                    ;         2
            ADC #b1 STA&70 ; 0 2 3 = 5
    LDA #a2 ADC #b2 TAY    ; 2 2 2 = 6
    LDA #a3 ADC #b3 TAX    ; 2 2 2 = 6
    LDA #a4 ADC #b4        ; 2 2   = 4
                           ; total  23 cycles

If you're adding constants, you might as well load each byte of the result directly, when you need it. (I can't tell where the LSB is coming from in this code - perhaps it isn't a constant? - this example doesn't resemble any code I've ever had to write.)

Perhaps the code is intended to be modified at runtime, but then you'd then still want one of the operands loaded from memory, I think (otherwise why not just precalculate the results?), and I've generally found the (fairly substantial) fixed expense not to be worth it anyway.

Anyway, overall I think you're being a bit unfair to the x86 with this comparison.