Ion drive systems use a magnetic coil assembly to accelerate high energy plasma through a vectored thrust system. In the pre-warp era plasma was generated by stripping electrons of light weight gasses before accelerating it with the magnetic coil assembly to produce thrust. While these particles can approach relativistic speeds the acceleration on the vessel was very low compared to modern Impulse Drive systems and could take days or weeks to reach maximum velocity and vessels had to have a fairly low mass.
In more modern ion drive systems (post-warp era) a fusion reactor or matter/antimatter reactor can be used to produce much high energy plasma allowing for vessels with higher mass. Still, the magnetic acceleration assembly limits how fast a vessel with ion drive can accelerate. Because of modern ways to produce high energy plasma, an ion drive can also be used for thrust inside an atmosphere when assisted by an anti-grav system.
Speed
Ion drives can reach a maximum velocity between 0.01c and 0.05c in open space. They can take hours or days to reach this velocity.
Relativistic Concerns
Ion Drives contain no way to counter the effects of time dilation at high velocities. This makes them a poor choice for speeds that approach the light speed.