Manual:$wgMaxInterlacingAreas/en

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.