Do you have a Kona or Decklink?
The 1440 “thin raster” frame size is more processor intensive than full raster 1920 as it must be scaled up on the fly. Kona cards do this in hardware which takes the burden off the host processor.
You might also try turning off realtime scopes if you have that turned on. I have seen that help with DVCPRO HD, which is also needs to be scaled.