Half-Async/Half-Sync vs Leader-Follower concurrency pattern

I am trying to understand the performance overhead between Half-Async/Half-Sync and Leader-Follower concurrency pattern. And was referring to this paper: https://www.dre.vanderbilt.edu/~schmidt/PDF/OM-01.pdf

Specifically, I could not understand the Half-Async/Half-Sync performance overhead. It says that:

Data exchange overhead
----------------------- 
When exchanging data between the synchronous and asynchronous
layers, the queueing layer can incur a significant performance overhead due to context 
switching, synchronization, cache coherency management, and data-copying overhead.

Can someone help with these 3 points in this context:

• context switching
• synchronization
• cache coherency management
• data-copying overhead

And, apart from the data-copying part, other overheads still exist in the Leader/Follower pattern right?