Does COPY FROM sidestep the WAL? My (perhaps incorrect) understanding is that pg writes such data to a table file then uses a small transaction to make it live.
In my understanding, as compared to a multi-row insert, a COPY statement is (1) a lot more efficient to parse/process, (2) uses a special bulk write strategy that avoids churn and large amount of dirty pages in shared buffers.
As compared to multiple single row inserts another benefit is that you're writing a single transaction ID, instead of a lot of individual transaction IDs (causing less freezing work having to be done by autovacuum, etc.)