The fastest turnovers are planned before the tenant moves out. If you know the move-out date, book the painter for the day after, and walk the unit, or send photos, beforehand so the scope and price are set before the keys come back.
Standardize your colors. Using one wall color, one trim color, and the same sheens across your units makes the next turnover faster and lets you touch up instead of repainting whole rooms. Keep a note of the products and colors for each property.
Expect patching. Nail holes, anchors, furniture scuffs, and door-handle dents are normal. Patched spots need priming before paint or they'll show through as dull patches, so make sure your painter includes that in the estimate.
Finally, put the work in the right order: repairs first, then paint, then cleaning. Painting after the cleaners have been through means paying for a second clean. Doing paint and cleaning with the same crew in one visit is often the quickest way to get the unit ready to list.