Manual:$wgUploadThumbnailRenderMap

Thumbnail settings: $wgUploadThumbnailRenderMap
An array of thumbnail widths to be rendered at upload time.
Introduced in version:1.25.3 (Gerrit change 157157; git #623d7789)
Removed in version:still in use
Allowed values:(array)
Default value:[]

Details edit

When defined, is an array of thumbnail widths to be rendered at upload time. The idea is to prerender common thumbnail sizes, in order to avoid the necessity to render them on demand, which has a performance impact for the first client to view a certain size.

This obviously means that more disk space is needed per upload upfront.

See also edit