The Balkan campaign didn't delay Barbarossa by a single day. The Balkan campaign was in fact an amazing achievement, completed far quicker than any of the Nazi's military thinkers or high command thought possible. Hitler had always been over eager and had a tendency to place invasion dates way too early. Hitler had told his generals he wanted to invade France the same year that Poland fell and had to be talked into placing the invasion date back into 1940. Barbarossa's date was no exception; the original date of May 15, 1941 was way too optimistic and didn't take into account that the thawing of Russia's winter had turned the roads into muddy tank traps, and the rivers were overflowing. The German's were also having logistical trouble moving their men into position and building the necessary air bases in Poland. The Balkan campaign also became necessary after Yugoslavia pulled out of the Tripartite pact leaving Hitler's southern flank exposed.

Also, Hitler wanted to attack as quickly as possible; mainly because he saw how bad the Red Army preformed against the Finns and he was determined to attack before its reorganization was complete. Hitler assumed with how weak the army was, that the Soviet Union would collapse in a few weeks, but he also knew they wouldn't be that weak for ever and so he decided to seize the moment and attack.

Source:

Second World War by: John Keegan

Russo-Finnish war: William R Trotter

Cataclysm: War on the Eastern Front: Keith Cumins