Why does this code lag the server to the point where roblox has to timeout even with a wait?

you can change it with this:

db_value.Value = db
repeat wait()
db_value.Value =  db_value -1
print(db)
until db_value.Value == 0
end

This is because you're not doing anything inside of the repeat-until statement.

The code you want to be repeated until it's condition is true has to be inside of the repeat-until statement.

repeat
    print(I'll be printing forever. True will never equal false.)
    wait()
until true == false

The code above will print forever since true will never equal false. That was an example of how you could use it.

You could also use it like this, which will fit your needs perfectly.

db_value.Value = db
repeat 
    wait(1)
    db_value.Value = db_value.Value -1
    print(db)
until db_value.Value <= 0
print("db_value equals 0 or less.")

If you want to read more about repeat-until statements then feel free to look at the pil.

Reading from what @Harry_TheKing1 said basically, your script renders so fast because there's no wait and you didn't put the below code after the until function after

db_value.Value = db
repeat 
wait(.5)
db_value.Value =  db_value -1
print(db)
until db_value.Value == 0
end