Summary form only given. Overlapping communication with computation is a well-known technique to increase application performance. While it is commonly assumed that communication and computation can be overlapped at no cost, in reality, they do contend for resources and thus interfere with each other. Here we present an empirical quantification of the interference rate of communication on computation. We measure this rate on a single processor communicating with both local and remote processors via Java sockets. Among other results we find that the computation rate can suffer by as much as 50%, and that the reduction is approximately proportional to the communication rate. We conclude that interference deserves further study.