LotR -- Book 3 - Chapter 09 - Flotsam and Jetsam

[the phantom]

It seems that the Ents can resist

Saruman may not have known how to appeal to them.

they would have taken their time to consider Saruman's words, thus negating their immediacy and therefore, possibly, their "magic"

No, no, and no.

From ROTK, Many Partings (Gandalf speaking to Treebeard)-

He had the poison of his voice, and I guess that he persuaded you, even Treebeard, knowing the soft spot in your heart.

Ents couldn't resist. After Gandalf left, Treebeard was "left alone to talk with him" and Saruman talked his way out of captivity.