Japan and South Korea sit on some of the shortest ocean lanes out of China, with transit times from the main Chinese gateways often measured in days rather than the weeks typical of transpacific or Asia–Europe routings. That short transit changes the economics that apply to longer lanes — the cost gap between air and sea freight narrows in relative terms, because sea freight is already fast enough that air's speed advantage matters less for most cargo.
Short-sea frequency is also higher than on long-haul lanes, with multiple sailings per week on the main China–Japan and China–Korea city pairs, which gives more flexibility on booking timing and reduces the capacity-crunch effect that peak season has on longer routes.
Busan, in South Korea, also functions as a transshipment hub for cargo moving onward to other destinations, similar to Singapore's role in Southeast Asia, which is worth knowing if a booking is routed through Busan rather than sailing direct.
For an importer used to planning around four-to-six-week transpacific or Asia–Europe transit times, the main adjustment on these lanes is simply recalibrating expectations — booking lead times can be shorter, and LCL consolidation cycles tend to run faster given the shorter overall transit.