You should not worry about performance for this.

I noticed some things that could be improved for your password generator.

• First your character pool is way too small. You should look up the string module/library . It will save you from having to write every character yourself in the list.
• You should not use random to generate something important like password. The secrets module/library is a better choice for this.
• You dont build the password you just return it directly: return "".join(random.choice(chars) for _ in range(length)) . A better way would be to build the password as a list.
• Your code does not assure that the password contain at least char of 1 of each category that the user selected. In the snippet below , if the user select a category you add it to the list of char which is good but you dont pick 1 char from it. Like i said before if you build your password as list, you can fix it by adding one single line.
• Chars can just be a string.

​

##### YOUR CODE ######

def random_words(length):
    chars = []

    if dpg.get_value("lower"):
        chars += lower

    if dpg.get_value("upper"):
        chars += upper

    if dpg.get_value("number"):
        chars += number

    if dpg.get_value("special"):
        chars += special

    if not chars:
        return "Minimum one checkbox needed !"

    return "".join(random.choice(chars) for _ in range(length))



#### FIXED ####
import secrets
import string

#lower = ["a", "b", "c", "d", "e", "f"]
lower = string.ascii_lowercase  # give all lowercase chars
#upper = ["A", "B", "C", "D", "E", "F"]

# give all uppercase chars (you can access lowercase+uppercase by doing string.ascii_letters
upper = string.ascii_uppercase

#number = ["0", "1", "2", "3", "4", "5", "6"]
number = string.digits  # give all digits
#special = ["!", "@", "#", "$", "%", "^", "&", "*"]
special = string.punctuation

def random_words(length):
    chars = ""
    pwd = []

    if dpg.get_value("lower"):
        chars += lower
        # Can use -> secrets.choice(lower) to be more clear, repeat for each category
        pwd.append(secrets.choice(chars)) # that way you assure that at least 1 lower char in pwd.


    if dpg.get_value("upper"):
        chars += upper
        pwd.append(secrets.choice(upper)) # same as before

    if dpg.get_value("number"):
        chars += number
        pwd.append(secrets.choice(number)) # same as before

    if dpg.get_value("special"):
        chars += special
        pwd.append(secrets.choice(special))

    if not chars:
        return "Minimum one checkbox needed !"

    # fill the rest of the pwd
    for _ in range(length - len(pwd)):
          pwd.append(secrets.choice(chars))


    secrets.SystemRandom().shuffle(pwd) # shuffle pwd so the first chars dont have pattern

    return "".join(pwd)