In short this makes TCP over loopback go wroom wroom.

Delaying ACKs on loopback is the cause that in certain situations no
packets flow until we hit a timeout (around 1 second). This mainly affects
request/respose protocols (like iscsi).

-- 
:wq Claudio

Index: tcp_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.235
diff -u -p -r1.235 tcp_input.c
--- tcp_input.c	20 Jul 2010 15:36:03 -0000	1.235
+++ tcp_input.c	23 Sep 2010 14:20:07 -0000
@@ -177,10 +177,12 @@ do { \
  * We also ACK immediately if we received a PUSH and the ACK-on-PUSH
  * option is enabled.
  */
-#define	TCP_SETUP_ACK(tp, tiflags) \
+#define	TCP_SETUP_ACK(tp, tiflags, m) \
 do { \
 	if ((tp)->t_flags & TF_DELACK || \
-	    (tcp_ack_on_push && (tiflags) & TH_PUSH)) \
+	    (tcp_ack_on_push && (tiflags) & TH_PUSH) || \
+	    (m && (m->m_flags & M_PKTHDR) && m->m_pkthdr.rcvif && \
+	    (m->m_pkthdr.rcvif->if_flags & IFF_LOOPBACK))) \
 		tp->t_flags |= TF_ACKNOW; \
 	else \
 		TCP_SET_DELACK(tp); \
@@ -1115,6 +1117,8 @@ after_listen:
 			tcpstat.tcps_rcvpack++;
 			tcpstat.tcps_rcvbyte += tlen;
 			ND6_HINT(tp);
+
+			TCP_SETUP_ACK(tp, tiflags, m);
 			/*
 			 * Drop TCP, IP headers and TCP options then add data
 			 * to socket buffer.
@@ -1126,7 +1130,6 @@ after_listen:
 				sbappendstream(&so->so_rcv, m);
 			}
 			sorwakeup(so);
-			TCP_SETUP_ACK(tp, tiflags);
 			if (tp->t_flags & TF_ACKNOW)
 				(void) tcp_output(tp);
 			return;
@@ -2060,7 +2063,7 @@ dodata:							/* XXX */
 #endif
 		if (th->th_seq == tp->rcv_nxt && TAILQ_EMPTY(&tp->t_segq) &&
 		    tp->t_state == TCPS_ESTABLISHED) {
-			TCP_SETUP_ACK(tp, tiflags);
+			TCP_SETUP_ACK(tp, tiflags, m);
 			tp->rcv_nxt += tlen;
 			tiflags = th->th_flags & TH_FIN;
 			tcpstat.tcps_rcvpack++;