It would be nice to know which model you went with performance pro has all sizes and you may have just gone too big period.worry the flow is too strong that it will end up stirring up debris at the bottom of the pond and may not be a good environment for aquatic plants.
An 3/4 inch is only good for 11 gpm a 1.5 is good for 35 gpm and a 2 inch is over 80 gpm . So even though 1.5 is only twice as large as 3/4 it puts out 3 times as much water . And 1.5 to 2 inch that is only a 1/2 inch larger than 1.5 puts out twice as much at 80 over 35gpm .
What size is the discharge on the pump? I'm betting it's 2 inch meaning your pump is probably over 4000 gph and seeing as you have ONE 1.5 line leading from the pump that can only handle 2100 gph so to push half of of you bound up pressure again if your 4000 gph that's 1900 gph that's being pushed to a pipe that can't handle it at low pressure and is pushing it to a higher psi adding head pressure. And seeing as my scenario is half its power being choked the pump will not last as long. Again this is IF my numbers are close to what pump you have if it is a higher gph it's even worse.
Solution can come when we know how many gph your pump is. Unfortunately you didn't need a high head pump. But let's see what you got.