The LSS uses a background thread called the "Lazy writer" to write dirty segments in the segment cache to disk. Therefore the thread that opens a transaction and writes some serial elements will typically only write the byte stream to a buffer in memory, allowing it to complete the transaction very quickly without blocking on I/O.
It is important to understand that ending a transaction implicitly commits it, but only in the sense of defining an atomic unit of work. The actual data is written to disk in the background.
When writing larges amounts of data, the segment cache can become full of dirty segments, and in that case it is possible for the thread performing a transaction to block on I/O.