Why was Truman's response to events in Europe so hesitant? (in regards to the origins of the cold war)
I think Truman responded correctly, and fairly quickly, but let's also remember that he had a steep learning curve to deal with, having inherited the job from FDR in 1945 right before we obtained an atomic bomb and while tensions between the Soviet Union and the US were just beginning to rise.
He had good advisers, and realized very quickly (with some help from George F. Kennan) that the Soviets were determined to spread communist government around the globe, and they would only respond to forceful containment. He quickly adopted the Truman Doctrine, and within two ears had formed NATO and protected West Berlin, and later South Korea. I'd say we have to give him a little more credit at being an effective cold warrior.
I am not sure that this is quite fair to Truman. After all, he did put the Truman Doctrine into effect, helping to prevent communism from spreading into Greece and Turkey. It is also hard to say that the Marshall Plan was a hesitant action. The formation of NATO and the Berlin Airlift also seem like forceful actions in my mind.
To the extent that he was hesitant, I would say that it was because there was no desire on just about anyone's part (in the US) to get into a war with the Soviet Union over a bunch of countries in Eastern Europe. I think that Truman saw that those countries were much more important to the Soviets than they were to us and acted accordingly.