Is it possible to control the speed of the fan on my central A/C unit?
There are numerous instructional videos on how to change the air speed CFM on an airhandler, so YES.  Rather than a switch, it is achieved by the wiring.  There will be a chart and a wiring diagram on the unit that will direct you.  One speed of air does not match all configurations.  A specific CFM on a 3 ton unit with a single trunk line feeding a shotgun house might not be the same as a 4 ton unit with a different duct set up, yet the air handler itself might be the same.