With Replication Server 15.5, the distributor (DIST) thread reads SQL statements directly from the Stable Queue Transaction (SQT) thread cache. This reduces the workload from SQT and the dependency between the two, and improves the efficiency of both SQT and DIST.
See “Enhanced distributor thread read efficiency” in Chapter 4, “Performance Tuning” in the Replication Server Administration Guide Volume 2.