70 seconds sounds like a lot of thumb twiddling to me, but my LX-90 is pre-GPS, and I've never measured slew times.
I know my dad's LX-200 GPS takes what seems like forever to complete a slew, so maybe that's the nature of having GPS in control.
How is the accuracy after the slew?
As others have suggested, "quiet slew" mode sounds like obvious culprit but you've confirmed that's not the issue.
And, I'm guessing this isn't a "high precision" issue either?
How were the slew time before you calibrated motors and trained the drives?
My last though would be your power supply, but you said you're plugged into an outlet.
Is is powered by a long (say 100') extension cord?