Edges in networks often represent transfer relationships between vertices. When visualizing such networks as node-link diagrams, animated particles flowing along the links can effectively convey this notion of transfer. Variables that govern the motion of particles, their speed in particular, may be used to visually represent edge data attributes. Few guidelines exist to inform the design of these particle-based network visualizations, however. Empirical studies so far have only looked at the different motion variables in isolation, independently from other visual variables controlling the appearance of particles, such as their color or size. In this paper, we report on a study of the influence of several visual variables on users' perception of the speed of particles. Our results show that particles' luminance, chromaticity and width do not interfere with their perceived speed. But variations in their length make it more difficult for users to compare the relative speed of particles across edges.