First, we can lift the enc.decodeMap nil check out of the loop.
Second, we can make it clear to the compiler that 'in := src[0]' doesn't
need a bounds check, by making len(src)==0 a single if check that always
stops the loop. This is by far the largest speed-up.
Third, we can use a dst slice index instead of reslicing dst, which
removes work from the loop body.
While at it, we can merge the two 'switch dlen' pieces of code, which
simplifies the code and doesn't affect performance.
name old time/op new time/op delta
DecodeString-8 80.2µs ± 0% 67.5µs ± 0% -15.81% (p=0.002 n=6+6)
name old speed new speed delta
DecodeString-8 163MB/s ± 0% 194MB/s ± 0% +18.78% (p=0.002 n=6+6)