Slew Rate related EMI: How relevant is it for motor control circuits ?
You certainly don't want a slew rate so slow that it'll have a significant impact on available duty cycle at 20kHz, there is a switching loss in the output FETs every time they are not either in full conduction or fully off, so minimizing the switching time minimizes the power loss and heating.
  On similar controllers, I've found a 0.5 - 1us rise/fall time typically is an acceptable compromise between switching losses and EMI generation, but it depends greatly on the load, component and trace placement, and the interaction of the various other components, including the recovery time of the freewheeling diodes - the body diodes that the FETs offer are usually pretty slow and can cause issues.

Distance between the EMI sources and any sensitive components won't guarantee immunity either, conducted noise can easily affect anything connected to common lines, and placement of decoupling caps or filters may be needed, but I doubt that there'd be many issues with a controller of the specs you quote. Just be liberal with decoupling caps, they're a cheap insurance.

These modules look to be aimed towards fairly low frequency operation, the on/off delay times are going to limit the range of duty cycles anyway - FETs intrinsically have a delay of just tens of nanoseconds, and with a microcontroller driving them you can play with the dead time that is present between the gate driving signals to fine tune the timing, but on these modules that all appears to be built in and fixed, and the delays that the control circuits introduce are dependent on the selected slew rate.