FabFilter

Add a new Aux track to Presonus Studio One. Add Pro-R to this track. Then select any other track in your project and a new Send to the Pro-R. Open the performance meter and the CPU is hitting 100%. Do this for a second Send and this pretty much brings Studio One to it's knees screaming in pain.

Config:

Windows 10
i5 CPU 4Gb Ram on Asus board
Focusrite Scarlett 6i6, buffers 480/480 in S1

I assume that when you say "Aux track" you mean an FX channel (going by S1 terminology here).

Since I had a track at hand, where I have an FX channel with Pro-R on it, I just tried sending more tracks to it using sends. This made no difference in CPU performance (as shown by S1) on my system.
Pro-R stays at 3-5% CPU usage no matter how many channels I send to the FX channel with it on.

Using Windows 10 and Studio One 3.3.4 Professional (64-bit).
M-Audio/Avid Fast Track C400 64-bit ASIO using 1024/1024 buffer setting. 32-bit/44 kHz (song being 24/44).
i7 4930k 6-core CPU running at 4.2 GHz and 32 GB DDR3 RAM (2133 MHz).