Extremely Long Render Times in Audition CC 2017

I have a multitrack session with six tracks and a stereo master track. Each track (including the master) has an Audio Unit plugin effect, and the six audio tracks have the "pre-render" box checked. The session is just over 30 minutes long.

When I select "Export > Entire Session..." and try to export as 44.1kHz, 16-bit Stereo (the original session is 48kHz, 32-bit Stereo), the render time estimates that it will be approximately one hour! If I instead choose to "Mixdown to New File", the render time is approximately one-third of this.

Is exporting an entire session broken? I'm currently using Audition 2017.0.2 but have experienced this problem going all the way back to the 2015 versions. Even the shorter, 20-minute mixdown seems excessively long for the amount of processing being done.