The Terumat Ha’deshen (Rav Yisrael Isserlin, 1390-1460), in a famous responsum (139), writes that there is a Misva Min Ha’mubhar (especially high standard of performing the Misva) to swallow the Ke’zayit of Masa at the Seder all at once. Meaning, one chews the Masa as he puts it in his mouth without swallowing, and only after the entire Ke’zayit is in his mouth, he swallows all of it at the same time. The Terumat Ha’deshen bases this view on a comment of the Mordechi.
There is a debate among the Halachic authorities as to whether the Shulhan Aruch accepted this position of the Terumat Ha’deshen. Hacham Bension Abba Shaul (Israel, 1923-1998) understood the Shulhan Aruch’s comments (Orah Haim 475:1) as suggesting that one should ideally swallow the Ke’zayit all at once, following the Terumat Ha’deshen’s position. Other Aharonim similarly understood the Shulhan Aruch’s ruling in this fashion, including Rav Zalman of Liadi, the Aruch Ha’shulhan, and the Magen Abraham. Hacham Ovadia Yosef, however, in Hazon Ovadia – Teshubot, interpreted the Shulhan Aruch’s comments differently. According to his reading, the Shulhan Aruch did not accept the Terumat Ha’deshen’s ruling, and there is no reason at all to try to swallow the entire He’zayit all at once. As long as one eats the entire Ke’zayit without interruption, within the period of "Kedeh Achilat Paress," he has fulfilled the Misva at the highest standard. Preferably, one should eat the Ke’zayit within four minutes, but even if he eats it within nine minutes, he has fulfilled the Misva properly. The Shulhan Aruch rules (475:6) that if one eats the Ke’zayit over the course of a period that extends longer than "Kedeh Achilat Paress," then he fulfills the Misva only on the level of "Be’di’abad" (after the fact). But as long as one eats the Ke’zayit within 4-9 minutes, he has fulfilled the Misva properly, at the highest level.
Hacham Ovadia further notes that several Halachic authorities did not observe the Terumat Ha’deshen’s practice. These include the Maharil (Rav Yaakov Halevi Moelin,1365-1427), the Rabbi of the Terumat Ha’deshen, and the Eliya Rabba (Rav Eliyahu Shapiro of Prague, 1660-1712). Additionally, the Aruch Ha’shulhan wrote that one does not have to observe this practice. In light of all this, and given the concern that attempting to swallow an entire Ke’zayit all at once could pose a serious hazard, Hacham Ovadia writes that as a rule, this practice should not be encouraged. If a person is certain that he can swallow the Masa in this fashion safely, and he will not creating a Hillul Hashem (defamation of G-d) by appearing gluttonous or unbecoming, then he may observe this practice and "Tabo Alav Beracha" (he is worthy of blessing). In general, however, one should eat the Masa in the normal manner of eating, ensuring to complete the Ke’zayit within a period of 4-9 minutes.
Summary: One should eat the Ke’zayit of Masa at the Seder in the normal way he ordinarily eats, ensuring to complete the Ke’zayit within a period of 4-9 minutes.