Probably your USB sends are set to ‘pre’. You want them set to ‘post’.

In the channel settings, for every channel you use, go to the routing tab and set the ‘digital send options’ to ‘post’. That should do it

Thanks! Is that the knob that says DAW at the top, with the pink button? It's set to Off. I'll check it later - rendering a 1-hour project just now and it's exporting at 1.2x. Ugh!

Yes, on the console that will do the trick

As for the slow rendering that may be due to your signal flow. When you incorporate you mixer’s signal processing in your mixes it will slow rendering down to (near) real time. To make full use of your computer’s processing speed all processing needed for the rendering needs to be ‘in the box’.

Great! It's working now with the EQ.

About the rendering: I record some audio into Studio One using the SL3 as an audio interface, but once it's in the box, how would the mixer slow down the render?

When Studio One sees any ‘live’ input (open channels coming in e.g. via USB) during rendering then things will slow down to real time processing. I haven’t tested this in detail (never really had an issue with rendering speed) but others may have. Could be a matter of making sure that none of the channels are armed during rendering