The two main sets of changes in 5.1.5 are enhancements to Frontier's webserver and bug fixes for guest databases. Faster, more reliable serving Overall threading performance was improved.
Threads start up more quickly and cooperate more effectively.
Threading during tcp operations in particular has been improved on both Mac and Windows. On the Mac, tcp.writeStream can handle very large blocks efficiently, chunking internally in a hightly threaded fashion. The problem where a client of a Mac server would get a broken connection error at the end of a large download is fixed. On Windows, accepting an incoming listen is more efficient, and DNS operations are more asynchronous. Overall server performance is significantly enhanced on both platforms.