You want to set the DynamicReflection_DownsampleFactor to 1
the other one that can have an effect is the DynamicReflection_HaltAfterFirstReflectingSurface
(this does not do what the name indicates it copies a buffer the size of your resolution when the game signals reshade that it is done drawing the map buut this can actually happen multiple times a frame which will result in more than one copy that can also slow it down a bit but with no filtering it should not be that significant)
