I don't get it. Why do so many people think that saying "amen" longer is better? (Rhetorical question!) Presumably because the long amen helps them think through all of the appropriate meditations and make the amen more meaningful. But the simple fact is that a long amen is not halakhically a good thing.
Shulchan Arukh, Orach Chaim 124:8:
ולא יענה אמן קצרה אלא ארוכה קצת כדי שיוכל לומר קל מלך נאמן ולא יאריך בה יותר מדאי לפי שאין קריאת התיבה נשמעת כשמאריך יותר מדאי.In another example of "the golden mean", the Shulchan Arukh rules that one should say a medium-length amen. It should take the amount of time takes to say the phrase "Kel melekh ne'eman", which is about 2-3 seconds. A longer amen is difficult to recognize. Perhaps people with 5+ second-long amens assume that listeners can understand what they are saying. But if that is so, what is the case that the Shulchan Arukh says not to do? A minute-long amen? I find that hard to believe.
One should not say a short amen but a slightly long one, [long enough] that one can say "Kel melekh ne'eman". And one should not say it too long because the word is not heard when one lengthens it too much.