There is a known bug in regards to the speed commands sent by radar controllers that you should know about either as a pilot or controller.
On Radar the speed shown is in GS (Ground Speed), but when a radar controller sends a specific speed command, it is always in reference to your IAS (Indicated Airspeed). The bug though is that the speed commands being sent are being sent in GS, so to clear up confusion, here’s an example.
For example, let’s say the pilot is flying at 200knts IAS and 235knts GS.
When the radar controller sends a speed command to increase speed to 220knts, the message played will say “decrease speed to 220knts”. This is not supposed to happen. The system thinks the pilot is at 235knts not 200knts (235knts GS to 220knts IAS), when they actually need to increase their speed because the controller is not referring to their GS.
So just know that IAS is what a radar controller is referring to when they are sending a speed command, and to ignore the system generated reduce or increase speed portion.