Manual:$wgThumbnailBuckets/en

Details
When  is defined, MediaWiki will use intermediary thumbnails to speed up thumbnail rendering. must be set to a list of thumbnail widths; whenever a new thumbnail needs to be rendered, MediaWiki will find the smallest bucket which is larger than the target width +  but smaller than the original, and it will scale that thumbnail down to the target size instead of scaling down the original file. This is down recursively, i.e. if the intermediary thumbnail for that bucket does not exist, it will be created from the intermediary thumbnail for the next bucket.

This will increase thumbnail generation speed considerably, especially for large images, but it will cause some loss of quality as thumbnails are now the result of several chained lossy transformations instead of just one.

File handlers have to opt into using this feature via. For now only  supports it.