Is it going to rain and your windshield wipers are going back and forth tragically slow? This can have various causes. Your classic usually has an average age of 30 years or more. Have the correct wiper arms been installed? You never know, maybe the previous owner changed them?
Then you or the 'feel' wiper blades is not pressed too tightly against the window. It is also important that all electrical connections make a good 'connection'. This must be checked for both the wiper motor and the switch. It is of course also possible that the so-called wheel boxes where the windscreen wiper arms are mounted on, go heavy. That has - most likely - to do with hardened fat that the manufacturer put in more than three decades ago. To get to it, you will have to be under the dashboard in most cases. Just behind 'the clocks' on one side, behind the glove compartment on the other. If this does not help, it is wise to pick the wiper motor apart, starting where 'the wire' enters. There too, fat was stuffed in from the factory three decades or more ago and that too may have become 'hard'. It is sensible to remove such grease and apply new - more modern - grease. Finally, there could also be a problem with the carbon brushes. Replace for new ones. If this does not resolve the problem, there is only one remedy left, replacing the wiper motor for a better, new or newer one. If you do not do anything about it, then the day they give up completely is approaching and that only happens when it rains, because then the windscreen wipers need ...