Tunze
DOC Skimmer 9410 or
DOC Skimmer 9410 DC. Maybe the 9415 if you think you'll have more tanks on the system or that you'll upgrade to a larger system.
Assuming everything is set up right...
Check the air-intake where the hose connects to the pump to see if there's any blockage – hidden or obvious.
Could also be a worn out impeller.
I think it's a wash since the 75 gallon is A) not stocked so it will knock down your density rating and B) isn't even close to full, more than likely, so it's not going to contribute as much volume as it seems anyway.
For example, the Tunze
DOC Skimmer 9410 is the skimmer I'd recommend for that tank. It's rated for a lightly stocked system up to 265 gallons, or a SPS/heavy system up to about 160 gallons. Even adding the full nominal 75 gallons to the 100 gallon display (which is less than 100 if you haven't subtracted for rock, sand, etc.) doesn't chage the skimmer you'd select.....instead of a 100 gallon "heavy" system, you're working with a 175 gallon lightly stocked system.
Clear as mud? ;)
I'd stick with counting tanks that have livestock in them (display, frag tank, etc) and don't worry about accounting for the volume of the sump.