I have a very basic nested for loop iterates over a 30K long string. This loop at most must take 2-3 miliseconds but it takes 10 seconds on my iphone. This is an inexplicable time for a smart phone. Why could this be happening? Here is the code.

let cyph_1 : [Character] = my_array1 // 26 characters
let cyph_2 : [Character] = my_array2 // 26 characters
let de_cyphed = String(de_cyphee.prefix(de_cyphee.count - 8))
var temp_2 = ""
var temp_1 = "30K long string, not that long"
for k in 0 ..< temp_1.count
{
    var exists = false
    let some_char = temp_1_characters[k]
    for m in 0 ..< cyph_2.count
    {
        if (cyph_2[m] == some_char)
        {
            temp_2.append(cyph_1[m])
            exists = true
            break
        }
    }
    if (!exists)
    {
        temp_2 += "(some_char)"
    }
}
temp_1 = temp_2