The speed of Chazaras Hashatz & The problem of Amen Yesoma:
Answering Amen to a blessing contains a number of laws and restrictions. The proper answering of Amen is so severe that Ben Azaiy states in the Talmud that on one’s form of answer is dependent the longevity of his life. One of the Amen restrictions is that one is not to answer an Amen Yesoma.
What is an Amen Yesoma: There are two types of Amen Yesoma’s. 1) One is not to delay the Amen. Rather, immediately upon the person concluding the blessing he is to answer Amen. 2) One is not to answer Amen to a blessing that one did not hear and does not know its identity. In this Halacha we will discuss a third scenario in which the Poskim rule involves an Amen Yesoma.
B. The law:
Answering Amen after the start of the next blessing: Included in the definition and prohibition of an Amen Yesoma is that one may not answer Amen to a blessing if the person who said the blessing has already begun the next blessing. Thus, once the Chazan by Chazaras Hashatz begins the first word of Baruch by the next blessing in his repetition, it becomes forbidden for the congregation to answer Amen.
The law for the Chazan: Accordingly, it is an obligation upon the Chazan to make an interval between his conclusion of a blessing, and the start of the next blessing, in order so majority of the congregation can answer Amen prior to him beginning the next blessing. [In other words, he should not begin the next blessing until the congregation has finished answering Amen.] Unfortunately, due to our great sins, many people stumble in this, as when they Daven for the Amud, they quickly begin saying the next blessing immediately after concluding the previous blessing, and do not wait at all in between.
The law for the congregation: In the event that the Chazan does not fulfill the above and rather immediately begins the next blessing prior to the congregation having the ability to properly answer Amen, then some Poskim rule that the negligent Chazan does not have the ability to cancel the rights of the congregation to answer Amen to the blessings of the repetition, and hence if he is going too fast they may nevertheless answer Amen. However, most Poskim rule that the congregation is not the answer Amen to his blessings, as doing so would consist of an Amen Yesoma, and so is the final ruling. Accordingly, a Chazan who recites the repetition too quickly without enough time for the congregation to answer Amen in between blessings, the prohibition against nullifying an Amen is upon his shoulders.
 Admur 124:11; Michaber 124:8; Brachos 47a
 Brachos 47a
 Admur ibid; Michaber ibid; Brachos ibid
 The severity: Ben Azaiy stated that whoever answers an Amen Yesoma his children will be Yesomim r”l. [Brachos ibid]
 Admur ibid; Rama ibid; Abudarham ibid
 Admur 124:11; 215:2
 See M”B 124:37; Kaf Hachaim 124:49; Halacha Berurah [Yosef] 124:26
 Halachos Ketanos 1:55; Maharam Ben Chaviv in Kol Gadol 40; Yad Aaron on Tur 114; Mahariy Mulko 59; Birkeiy Yosef 124:9; Kesher Gudal 9:29; Shalmei Tzibur p. 89; Shaareiy Teshuvah 124:16; Chesed Lealafim 124:5; Kaf Hachaim Falagi 15:52; Ben Ish Chaiy Teruma 1:12; M”B 124:37; Shaar Hatziyon 124:43 that this applies according to everyone; Magen Giborim 124 in Shiltei Hagiborim 4, brought in Biur Halacha 124:11 “Vekodem”; Kaf Hachaim 124:49; Betzel Hachochma 5:17
 M”B 124:37; Shaar Hatziyon 124:43 that this applies according to everyone
 Halachos Ketanos 1:55; Toras Chaim Sofer 124:12; Even Yisrael 9:3
 Maharam Ben Chaviv in Kol Gadol 40; Yad Aaron on Tur 114; Mahariy Mulko 59; Birkeiy Yosef 124:9; Kesher Gudal 9:29; Shalmei Tzibur p. 89; Shaareiy Teshuvah 124:16; Chesed Lealafim 124:5; Kaf Hachaim Falagi 15:52; Ben Ish Chaiy Teruma 1:12; M”B 124:37; Magen Giborim 124 in Shiltei Hagiborim 4, brought in Biur Halacha 124:11 “Vekodem”; Kaf Hachaim 124:49; Halacha Berurah ibid
 Poskim ibid; Kesher Gudal ibid; Kaf Hachaim ibid