to use this, you do the following:

1. add this line to /etc/sysctl.conf:

kern.allowkmem=1

2. and then you can:

# pstat -dld net_cycles net_packets     
net_cycles at 0xffffffff822b93c0: 154766140
net_packets at 0xffffffff822b93c8: 1979

3. then you can work out the average cycles per packet.

Index: net/if.c
===================================================================
RCS file: /cvs/src/sys/net/if.c,v
retrieving revision 1.611
diff -u -p -r1.611 if.c
--- net/if.c	30 Jun 2020 09:31:38 -0000	1.611
+++ net/if.c	6 Jul 2020 00:23:03 -0000
@@ -908,10 +956,14 @@ if_ih_input(struct ifnet *ifp, struct mb
 		m_freem(m);
 }
 
+unsigned long net_cycles;
+unsigned long net_packets;
+
 void
 if_input_process(struct ifnet *ifp, struct mbuf_list *ml)
 {
 	struct mbuf *m;
+	unsigned long tic, toc, pkts = 0;
 
 	if (ml_empty(ml))
 		return;
@@ -932,9 +984,18 @@ if_input_process(struct ifnet *ifp, stru
 	 * lists and the socket layer.
 	 */
 	NET_LOCK();
-	while ((m = ml_dequeue(ml)) != NULL)
+	atomic_setbits_int(&curproc->p_flag, P_CANTSLEEP);
+	tic = rdtsc();
+	while ((m = ml_dequeue(ml)) != NULL) {
+		pkts++;
 		if_ih_input(ifp, m);
+	}
+	toc = rdtsc();
+	atomic_clearbits_int(&curproc->p_flag, P_CANTSLEEP);
 	NET_UNLOCK();
+
+	atomic_add_long(&net_cycles, toc - tic);
+	atomic_add_long(&net_packets, pkts);
 }
 
 void
Index: netinet/ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.348
diff -u -p -r1.348 ip_input.c
--- netinet/ip_input.c	12 Apr 2020 11:56:52 -0000	1.348
+++ netinet/ip_input.c	6 Jul 2020 00:23:03 -0000
@@ -216,6 +216,29 @@ ipv4_input(struct ifnet *ifp, struct mbu
 	KASSERT(nxt == IPPROTO_DONE);
 }
 
+uint16_t
+ip_cksum(const struct ip *ip)
+{
+	const uint32_t *w = (const uint32_t *)ip;
+	uint64_t s;
+	unsigned int i;
+
+	s = (uint64_t)w[0] +
+	    (uint64_t)w[1] +
+	    (uint64_t)w[2] +
+	    (uint64_t)w[3] +
+	    (uint64_t)w[4];
+
+	for (i = 5; i < ip->ip_hl; i++)
+		s += (uint64_t)w[i];
+
+	do {
+		s = (s >> 16) + (s & 0xffff);
+	} while (s >> 16);
+
+	return (~s);
+}
+
 int
 ip_input_if(struct mbuf **mp, int *offp, int nxt, int af, struct ifnet *ifp)
 {
@@ -267,7 +290,7 @@ ip_input_if(struct mbuf **mp, int *offp,
 		}
 
 		ipstat_inc(ips_inswcsum);
-		if (in_cksum(m, hlen) != 0) {
+		if (ip_cksum(ip) != 0x0000) {
 			ipstat_inc(ips_badsum);
 			goto bad;
 		}