How to change the way time slots are shown

You can set flexible timeline in Settings//Design//Logo, background and timeline//timeline type There are 4 types of timeline:

Slots weekly - shows available time in the intervals equal to your service duration for a week. Starts counting from the first available time.

Modern - shows available time in the intervals equal to your service duration for a day. Starts counting from the first available time.

Flexible weekly - shows available time in the intervals equal to your timeframe duration for a week. Starts counting from the first available time.

Flexible - shows available time in the intervals equal to your timeframe duration for a week. Starts counting from the first available time.

If you have services that have fixed start times, you can try using Appointment at fixed time plugin.