Installed a new heat pump 3-4 years ago after nordine unit had trouble. Only changed the outside portion of system not inside fan. System falls behind - one end of house especially, but whole house when temps get to about 90 outside will not get but 12-15 degrees cooler than outside even when unit runs all the time. Do we need a new fan or is it did to replacing with a high efficiency heat pump (as advised for energy savings)?

This is a fairly common question in the summer. There is a lot more to temp control than just fan speed . For example, is there shade on the house? Good wall and ceiling insulation? Good attic venting? Compressor in direct sun? Low E thermopane? Size of system rated for house size? If the system works well when temps are lower, you may have other issues as noted above.