Do you have yours set to unrestricted or update frames every 5 seconds...RDP may be different than other remote viewing apps since we know for a fact that teamviewer and the like increase cpu dramatically..
i see about 5% blue iris rdp open unrestricted
blue iris version 4.3.9.0
i7 - 3rd gen
windows 10
my windows 7 box is higher
blue iris version 4.5.0.0
xeon e3 gen 3 no HA
about 10% more
each one RDP'ed from a windows 7 to 7/10