Are you certain about this?
If you edit wxFlatNotebook.cpp, lines 771ff like this:
void wxPageContainer::OnPaint(wxPaintEvent & event)
{/*
wxBufferedPaintDC dc(this);
wxFNBRendererPtr render = wxFNBRendererMgrST::Get()->GetRenderer( GetParent()->GetWindowStyleFlag() );
render->DrawTabs(this, dc, event);
*/}
does the problem persist?
We have postulated many times what might be causing this, update_ui messages being one thing, but rather firing off too many. However, most of this is theory so far, no really hard evidence.
If you could prove that it is the actual tab drawing code which causes the slowdown (if it disappears after commenting out that section), this would be great. Then Eran would probably be able to fix this issue by inserting 2-3 lines of code to filter out surplus events.