I heard that it says in chazal the woman only has pleasure at the end ("gamar bia") of tashmish (relations). Is there any source/citation for this?
This question does have halachic consequences although not the ones originally written by the questionnaire
The gemoro in kiddushin has the question if during the act of bioh with A a woman accepted kiddushim (although highly unlikely) from B. The gemoro says she is married to B and not A because A only wanted the kiddushin to be 'chal' at gmar bioh. If she is now considered a 'zona b'rotson' with A who is not her husband and therefore must obtain a get from B and also perhaps be punished with death is beyond the scope of this answer.
The pnai yehosha says the gemoro could have also said the 'woman' only wants the kiddushin to be chal at gmar bioh. His proof elsewhere from a yerushalmi in Sotah chaper 4 I am not sure about. All the yerushalmi says is that the woman has rotson or pleasure at the end. This need not mean like the pnai yehshua says at gmar bioh but after a time although not at the beginning if she is forced.
There are four possibilities. They both want at the beginning then she would be married to A. They both want at the end. Then she would be married to B. If man wants at the end and the woman at the beginning again she would be married to B. The problem is if the woman wants at the end and the man at the beginning. Does the woman have a say in the matter. In other words does it make a difference to her. On that the pnai yehoshua says, it would be at the end and she would be married to B. Since it does make a difference to her.
This fully answers your question if a woman has more pleasure at gmar bioh. This was a difficult question to answer and took quite a bit of effort and contains my own chiddushim.