quickly可以表示“速度快”,与fast一样,可以与下列动词搭配:speak , read , talk , write , walk , run ,drive , pass , come , glance 等。例如:She speaks as quickly as her mother . 她说话像她母亲一样快。(赵振才编著《英语常见问题解答大词典》增订版)所以,“He spoke too quickly for me to understand. 他说得太快了,我听不懂。”是正确的,其中的quickly 也可以换成fast。