This study is provocative, but may not tell the whole story. It is not clear to me that productivity should fall in direct proportion to the size of the call. That’s a rough approximation, and captures the direction, likely. But I suspect a nonlinear effect. It’s basically a scheduling issue, and scheduling response rates are notoriously nonlinear.
It is complicated by the fact that a ship is usually only able to be scheduled at one terminal, even though another terminal may have excess capacity and be able to handle it exactly on time. That depends on the degree of cooperation possible among terminal operators. An interesting study would be to look at calls at a single port with multiple terminals and see how often there is a berth available for an ULCV but the specific ship cannot use it because it is required to use a different terminal.
I don’t see an easy way for port management or terminal management or ocean carriers and alliances to solve that one.
That said, I agree with the conclusion: it’s not mega-calls. I don’t think we should be blaming the mega container ships for the problems. Those ships will come, so ports need to innovate. A goal like 6000 moves in 24 hours is reasonable.
New analysis suggest that port productivity levels are dropping, but ultra-large container vessels are not at fault
Source: Productivity is declining at the world’s biggest ports, ‘but mega-boxships are not to blame’ – The Loadstar
cfmp_appendix-complete
Here’s a white paper from California State University Long Beach which was used in the deliberations. 
Freight Efficiency White Papers_Final Version
Supply Chain and Logistics 