One thing you should be aware of; lower fan speeds will normally yield lower exit port temperatures. The lower flow speed allows the air to "dwell" longer as it passes through the cooling coils thus maximizing transfer of the cooling effect. The higher speed seems to cool faster on the basis of volume flow but a thermometer at the exit port will show cooler air at lower fan speed.

That's not to say you don't have a problem but part of the difference you see is normal.

if the a/c system has been retrofitted to 134a you have now lost 30% efficiency. doing this and doing that will help very little. Bottom line is cars today with 134a are fitted with larger evaporators and condensers. Ebay sells 12oz cans or virgin r12 that is the best thing you can do to get it working good again