Personally, it was not a big problem for me that the fights go on for a long time.
The first fight may feel too long, but in such cases, the presence of dialogues in the battle helps well.
But at some point I realized that the goal of this fight is not so much to win, but to stay alive. And this is quite an interesting move, although perhaps not everyone will like it.
Perhaps you should have added some simple battle before the battle with the Serpent, so as not to immediately frighten the players with such a powerful battle, and let them first get acquainted with the abilities of the characters.
But in fact, this is a matter of perception of each person individually:)