The work done in a system is the sum of the kinetic and potential energy of the system. In the case of a particle in a field, we'll consider the particle as the system and the work done by a field as an external influence on the particle.
As such, if the work done by the field is zero, the final kinetic energy of the particle is constant. That means, if over the course of time, the particle is made to follow a circular path, at the beginning and the end of that path, the kinetic energy and thus the speed will be the same. However, during the course of the path, i.e. in the middle of the circle, the kinetic energy may vary.