The "hang" is mostly likely related to the IAC (Idle Air Control). The IAC doesn't close right away on deceleration. It slowly ramps down to a close with a "hang" somewhere slightly above idle before it returns to the idle settings.
I'm not familiar with QFI, but Holley's Terminator system has a full set of IAC parameters to tune this. IAC Hold Position, Ramp Decay Time, RPM Above Idle to Start Ramp, and RPM Above Idle To Re-enable Idle Control. You can read here for reference, but I don't know how it will translate to the QFI.
Terminator EFI hanging up on higher idle.
