Manual:$wgMaxInterlacingAreas

Description
Used to enable interlacing for a given MIME type, up to a given pixel limit (interlacing is relatively expensive).

For basic information on what interlacing is, see http://pooyak.com/p/progjpeg/.

Currently supported MIME types in core: , ,  . Interlacing will be done via ImageMagick, or for jpeg/png via VIPS if  is enabled.