You have posted the updateFader() function out of context so it is difficult to provide help. Using millis() for timing requires you so do actions in steps based on how much time has passed since something occurred. This allows other things to happen in the slack time between actions.
So, you cannot stay in a function like updateFader() for any appreciable time. Break what the function does into small steps, perhaps each moving the slider a little and do other things between the slider movements. Is the real problem the delay(1000); in the function ? If so then break that down into say 1000 1 millsecond steps and do other things in the slack time.
Please post what you tried as a starting point