The rate of fire is adjustable between around 8.5bps and 17bps.
The Ion's cycle time is made up of the board's dwell setting added to the "recharge delay". For any given dwell setting, the more delay is added, the longer the cycle becomes, and the longer each cycle is, the less cycles can be completed in a second. Consequently, your rate of fire is reduced.
So, once you have found the optimal dwell setting for your marker, you then need to adjust the delay setting to set your rate of fire. If you were to change dwell at a later date for some reason - for instance, fitting a different bolt or installing a QEV - your rate of fire would change, and you would have to re-adjust the delay setting.
You can find a calculation on this page at www.zdspb.com
to tell you what your delay setting should be in order to achieve the ROF you want at a given dwell setting: -