Why is WinZip slow when extracting some files?

Starting with WinZip^® 10.0, the Windows Attachment Manager service may slow the WinZip extraction process significantly.

When you download files from the Internet or receive attachments via Outlook or Windows Mail, Windows creates an alternate NTFS data stream for the downloaded/received file which contains Internet Zone information which is used by the Attachment Manager in determining how the file should be handled (e.g., whether it should be blocked altogether or be allowed to be opened with warnings, etc.)

Since Zip files are not (at least, by default) considered to be "high risk" by the operating system, they are not themselves blocked but, as they can serve as a vehicle for files that are considered "high risk" and, as WinZip is Attachment Manager aware, it honors the presence of the alternate data stream, if it exists, on the Zip file and passes it on to Windows for files you attempt to extract. This means that all files you attempt to extract are subjected to additional processing by the operating system and that files not blocked by Windows are extracted with the same Zone information as the Zip file.

You can work around the issue by doing the following:

Right-click on the downloaded file and click Properties.
In the General tab of the Properties dialog, click the Unblock button; this will remove the zone information and eliminate the Attachment Manager's involvement in the extract process.
Click OK to close the Properties dialog.

More information about the Windows Attachment Manager can be found at this Microsoft Knowledgebase article along with information on how to permanently disable preserving the Zone information in file attachments.