I scanned the same test data a second time to find out whether an antivirus uses any optimizations. Avast's time dropped fractionally to 27 minutes, similar to Avira's second scan of 24:41. But some apps only scan files if they're new or change, and that makes a huge difference. Bitdefender's second scan of my test data took 27 seconds, and Kaspersky managed 170 seconds.