Interference from the user (whether or not you use the computer during the scan).

Interference from other programs attempting to update (download/install) components from the Internet.

Interference from other security programs attempting to scan at the same time.

Whether the scanning engine stalls, hangs or freezes.

Other running processes and programs in the background.

Competition for and utilization of system/CPU resources by the scanner.

Whether external drives are included in the scan.

cab, archived, compressed, packed, email, etc) that are scanned.

Disk size and used capacity (number of files that have to be scanned).

What action has to be performed when malware is detected.

Type of scan performed: Deep, Quick or Custom scanning.

Options to scan memory, boot sectors, registry and alternate data streams (ADS).

Options to scan for rootkits, adware, riskware and potentially unwanted programs (PUPS).

The program itself and how its scanning engine is designed to scan: using a signature database vs heuristic scanning or a combination of both.

The speed and ability to complete an anti-virus or anti-malware scan depends on a variety of factors.

Windows defender scan time