Have you run DPC latency checker? This is a good way to check for any processes that are affecting real-time audio streams.
Which version of Windows are you running? Steinberg say their tests of the most up to date Windows 10 show a drop in efficiency.
It may be that 85% efficiency with 50% CPU is not a bad figure.I’ve read some people here say that while the ASIO load quickly goes high, it’s not necessarily linear - so you could keep loading your system and ASIO goes up some more, but cpu load starts to increase more quickly.
Have you looked at the Windows resource manager within task manager? See if the CPU load is evenly balanced between your cores. I had a problem where some video redraw tasks for some reason appeared to take place on one core out of six, so the CPU load was very unevenly spread.