I just ran a 3 second 29.97 1920×1080 Pro Res clip at 70% using Twixtor and it took 1 minute 10 seconds on a similar machine.
Please send us a project so we can take a look at how your FCP file is set up. If you could just reduce it to the sequences that are needed and the sequence that uses Twixtor (obviously since you are slowing it down there may be a nested sequence that is needed), or at least tell us which sequence to look at and the part that has Twixtor applied.
You can send the FCP project to techsupport (at) revisionfx (dot) com.
Pete