When you start scan verification, email verifier pro only creates one verification process in your server that handles email list verification in batch size which has been set in your settings. eg: 50 at a time. This takes 0.5 secs to 1.5 secs average to validate 1 email address to complete based on server to server connection and processing data. Email Verifier Pro default scan system will use SMTP requests to target uploaded lists mail servers through HELO response. Processing time can vary on several other factors like different server zone locations and if a target mail server is protected by a firewall/antivirus or blocks connection. In that case email verifier pro app have a 15 secs default time to wait for a connection to wait before terminating it to process next one from list.
Optional feature Inbox Checker use a different approach on top of default scan system. It sends dummy email from database directly to uploaded lists to cross check mailbox existence on top of default scan results by a SMTP & IMAP server. Inbox checker only available in List validation.