This paper reports a study about system response time (SRT) in communication robots that utilize human-like social features, such as anthropomorphic appearance and conversation in natural language. Our research purpose established a design guideline for SRT in communication robots. The first experiment observed user preferences toward different SRTs in interaction with a robot. In other existing user interfaces, faster response is usually preferred. In contrast, our experimental result indicated that user preference for SRT in a communication robot is highest at one second, and user preference ratings level off at two seconds. However, a robot cannot always respond in such a short time as one or two seconds. Thus, the important question is "What should a robot do if it cannot respond quickly enough?" The second experiment tested the effectiveness of a conversational filler: behavior to notify listeners that the robot is going to respond. In Japanese "etto" is used to buy time to think and resembles "well..." and "uh..." In English. We used the same strategy in a communication robot to shadow system response time. Our results indicated that using a conversational filler by the robot moderated the user's impression toward a long SRT.