这是think的宾语从句,且有since的用法,即前半部分为现在完成时,后半部分为过去式.
since I've heard anyone is good中 I've heard anyone is good意思是我听过的某个人不错的发言,而加上since之后指自从（上一次）听过某个人不错的发言,it's been a long time since I've heard anyone is good是指距离上一次听过的不错的发言已经有一段很长的时间了.整句话翻译过来就是我想我已经很久没听过这么好的发言了.