Yes, this is possible but it does need the post processor to support it.
From a quick test it looks like the Mach3 post does not currently support this function, but it can probably be modified or a new post created that does what you want.
In the attached sample the post I used appears to only support slowing down on arcs (M02/M03) but at least it shows the functionality is there. There may be another setting I have missed which will make it slow down on angles also.
The shape as shown is a rectangle with 3 holes, 2 radius corners, 1 chamfered corner & 1 square corner.
The NC code attached shows slowing down on the holes and radius corners but full speed on straights, chamfer & square corner.
The Parameters used are "Arc Slow Radius", "Arc Slow Factor" & "Arc Slow Minimum %"