I use SBCL on Linux and have noticed a problem when writing lots of data to socket streams created with usocket. In file backend/sbcl.lisp, the function socket-connect creates a stream as follows: (setf (socket-stream usocket) (sb-bsd-sockets:socket-make-stream socket :input t :output t :buffering :full :element-type element-type)) If you look at the code for sb-bsd-sockets:socket-make-stream in sbcl/contrib/sb-bsd-sockets/sockets.lisp, you'll see that it specifies a default value of T for the keyword argument serve-events: (defmethod socket-make-stream ((socket socket) &key input output (element-type 'character) (buffering :full) (external-format :default) timeout auto-close (serve-events t)) This means that SBCL streams created by usocket have a true serve-events property. When writing large amounts of data to several streams, the kernel will eventually stop accepting data from SBCL. When this happens, SBCL either waits for I/O to be possible on the file descriptor it's writing to or queues the data to be flushed later. Because usocket streams specify serve-events as true, SBCL always queues. Instead, it should wait for I/O to be available and write the remaining data to the socket. That's what serve-events equal to NIL gets you. Usocket should add :serve-events NIL to the call to socket-make-stream. bob