Background thread runs loader.next_sequence_pair() × n_batch while GPU trains on the current batch. sync_channel(2) double-buffer. Eliminates loader latency from the critical path at b=256 where 256 sequential next_sequence_pair calls took ~19ms/step (the entire step budget at 52 sps). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>