MIME (Multipurpose Internet Mail Extensions) is a standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. It allows emails to include multimedia content, making communication richer and more informative. In terms of performance, MIME can impact the speed and efficiency of email transmission. Larger attachments can slow down the sending and receiving process, and the encoding required for non-text files can increase the size of the email. Properly managing MIME types is essential for optimizing email performance and ensuring compatibility across different email clients.