If you can rig up a manometer and measure the pressure difference between the return & supply sides of the air handler, clean the parts you can easily access, then re-measure you can see if it made a significant difference. Use your best vacuum cleaner with the highest suction and a smaller detailing brush type tip to see if you can't really clear anything that has gotten between the fins. I you have any bent/flattened fins on the coil use a purpose-made coil-comb to gently open them back up.
But if it looks like this on the other side, you may be better off replacing it, since the condensation soaked into the crud will often have corroded the fins to the point of reducing their heat transfer rate:
If it looks like this under surface crud, it's clearly toast: