Without a Yugoslav coup, which is by no means a given, the Germans might be able to start Barbarossa early.
Two arguments against this:
(1) The late spring thaw, making the Russian roads a muddy mess;
(2) To quote an old soc.history.what-if post of mine:
***
Actually, 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. To quote Martin L. Van Creveld, *Hitler's Strategy
1940-1941: The Balkan Clue* (Cambridge University Press 1973), pp. 173-4:
"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."