Software packet forwarding has been used for a long time in general purpose operating systems. While interesting for prototyping or on slow links, it is not considered a viable solution at very high packet rates, where various sources of overhead (particularly, the packet I/O mechanisms) get in the way of achieving good performance. Having recently developed a novel framework (called netmap) for packet I/O on general purpose operating systems, we have investigated how our work can improve the performance of software packet processing. The problem is of interest because software switches/routers are widely used, and they are becoming inadequate with the increasing use of 1..10 Gbit/s links. The two case studies (OpenvSwitch and Click userspace) that we report in this paper give very interesting answers and insights. First of all, accelerating the I/O layer has the potential for huge benefits: we improved the performance of OpenvSwitch from 780 Kpps to almost 3 Mpps, and that of Click userspace from 490 Kpps to 3.95 Mpps, by simply replacing the I/O library (libpcap) with our accelerated version. On the other hand, reaching these speedups was not purely mechanical. The original versions of the two systems had other limitations, partly hidden by the slow packet I/O library, which prevented or limited the exploitation of these speed gains. In the paper we make the following contributions: i) present an accelerated version of libpcap which gives significant speedups for many existing packet processing applications; ii) show how we modified two representative applications (in particular, Click userspace), achieving huge performance improvements; iii) prove that existing software packet processing systems can be made adequate for high speed links, provided we are careful in removing other bottlenecks not related to packet I/O1.