There's a blower motor resistor that is very prone to failure, but I thought the symptom was that the fan switch wouldn't work on anything except high. Some food for thought but I think I just talked myself out of this being your problem.
If all that fails, I installed electric window regulators out of an '85 XLT into my '86 CC. If I decide I need more air flow, I can push a handy switch next to my knee and get a little more wind. I almost put in a sliding back glass, but if you think texting and driving is bad you oughta see a redneck opening a back glass in a CC going down the road. But seriously, you got leaves or something in the duct work or on the condenser or evaporator (I can't ever remember what it is in the dash). Consider putting HVAC tape around the seems on the blower box too.
Many older vehicles slowly leak out refrigerant. So not sure if you are saying it blows less cold, in which case adding some refrigerant might solve.
Or literately you mean the amount of air coming out your vents is less. (As in less windy in the cab when the fan is on.). If so folks above had some good ideas. Also I recall there are some vacuum servo’s that help direct air flow. So perhaps read up on that issue, but I have never looked into that, so no expert.
Low pressure port on drier could have failed. Evaporator frosts over after its on for a bit and it stops pushing the air to you. Turn ac to "vent" to force it to shut off for a minute or two every 5 minutes and see if it works better or worse. Better and that switch on drier needs replaced, worse and it is likely leaves and junk in Evaporator clogging up air flow.