The SLA Progress Bar calculates the percentage of time between the ticket logged date and the target time. It does not take into consideration working hours.
This means if the ticket target time is within your configured working hours it will move faster than if the ticket target time carry’s over to the next working day.
For example if a ticket is created at 11am with a 4 hour SLA and your working hours are 9am – 5pm the SLA Progress Bar will progress to 100% in the same day. However if the ticket was created at 3pm the progress bar will move a lot slower but will continue do so overnight until the next working day at 10am.