Martin L. Van Creveld, *Hitler's Strategy 1940-1941: The Balkan Clue* (Cambridge University Press 1973), pp. 173-4, has argued that even apart from the Balkans operations
and the late spring thaw, equipment shortages would have made it difficult to start Barbarossa earlier than in OTL:
"Halder's diary bristles with references to various units, none of which had anything to do with the Balkans, and whose equipment and training were not yet complete as late as the end of May 1941. Long before the Yugoslav campaign was in sight an entry reads 'the conversion of tanks into underwater tanks will require 12 weeks.' Allow another few days for transportation and 'Barbarossa', for which these machines were vital, could not start before the first days of June...
"Throughout May Halder was recording the difficulties encountered in supplying such diverse units as 100th armoured brigade, 13th, 17th and 18th armoured divisions and 14th and 18th motorized divisions, units which may have had nothing in common except for the fact that they had not been connected in any way with the Balkan campaign.
"Indeed, it is hardly possible to open a history of any 'fast' unit that took part in the war against Russia without being struck by the belatedness with which it was supplied with its full motor vehicle park. Thus, the so-called Panzerzüge [freight trains adapted to carrying and rapidly unloading armor and materials for use in occupying bridges, strongholds, etc.]; before the Yugoslav coup it had been decided five of these should be constructed but they were not ready for transportation to the east until mid-June, after finallly receiving French supplies. The 10th motorized division did not receive its equipment until after marching east on 10 June, and even then the vehicles had to be collected piecemeal from...Germany, Belgium, Holand and France!...Indeed, the problems created by the general shortage of equipment, particularly motor vehicles, were not limited to only the 'fast'units. At the time of the German offensive against Russia, no less than 92--or 40 percent--of the army divisions had to be supplied, wholly or in part, with French material. Since the losses in material resulting from the Balkan campaign were extremely limited, it is quite clear that lack of equipment of all kinds would have prevented 'Barbarossa' from starting before the end of June even if neither 'Marita' nor '25' [the Greek and Yugoslav operations] had come into the world."
https://archive.org/stream/HitlersS... 1940-1941 the Balkan Clue#page/n179/mode/2up