Nesting in FCP is, to use a technical term, wonky. IMO a better, although less flexible route, would be to export a self-contained QT movie of the sequence(s) instead of nesting them. Obviously any changes made would require a new QT movie to be exported, but it would keep things running smoother.
-A
3.2GHz 8-core, FCP 6.0.4, 10.5.5
Blackmagic Multibridge Eclipse (6.8.1)