The $.when object is the only one, which uses array-preallocation

// ...
		// add listeners to Deferred subordinates; treat others as resolved
		if ( length > 1 ) {
			progressValues = new Array( length );
			progressContexts = new Array( length );
			resolveContexts = new Array( length );
			for ( ; i < length; i++ ) {
				if ( resolveValues[ i ] && jQuery.isFunction( resolveValues[ i ].promise ) ) {
					resolveValues[ i ].promise()
						.done( updateFunc( i, resolveContexts, resolveValues ) )
						.fail( deferred.reject )
						.progress( updateFunc( i, progressContexts, progressValues ) );
				} else {
					--remaining;
				}
			}
		}

// ...

since it is not verly likely that when gets passed thousands of parameters this pre-allocation doesn't save much space or will boost performance.

Therefore I propose to save a few bytes here and reduce the 3 pre-allocation lines to something like

			progressValues = [];
			progressContexts = [];
			resolveContexts = [];

I am willing to provide an PR if accepted...