If frames are "offloaded" (forwarded by the hardware switch fabric), the ifconfig counters of the per-port netdevs will not increase, otherwise they will.
Here's two test scenarios, first bridged, then routed between my laptop, a wrt3200acm and my desktop. Laptop was wired to lan1, desktop to lan2.
Bridged scenario:
root@wrt3200acm:~# ip link add br0 type bridge
root@wrt3200acm:~# ip link set br0 up
root@wrt3200acm:~# ip link set lan1 up
root@wrt3200acm:~# ip link set lan2 up
root@wrt3200acm:~# ip link set lan1 master br0
root@wrt3200acm:~# ip link set lan2 master br0
root@wrt3200acm:~# ifconfig lan1 | grep "RX bytes"
RX bytes:26480 (25.8 KiB) TX bytes:3062 (2.9 KiB)
root@wrt3200acm:~# ifconfig lan2 | grep "RX bytes"
RX bytes:3752 (3.6 KiB) TX bytes:7388 (7.2 KiB)
root@desktop:~# ip addr add 10.255.1.1/24 dev eth0
root@desktop:~# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 128 KByte (default)
------------------------------------------------------------
[ 4] local 10.255.1.1 port 5001 connected with 10.255.1.2 port 53724
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 1.09 GBytes 933 Mbits/sec
^Croot@desktop:~#
root@laptop:~# ip addr add 10.255.1.2/24 dev eth0
root@laptop:~# iperf -c 10.255.1.1
------------------------------------------------------------
Client connecting to 10.255.1.1, TCP port 5001
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[ 3] local 10.255.1.2 port 53724 connected with 10.255.1.1 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 1.09 GBytes 934 Mbits/sec
root@laptop:~#
# Traffic counters only increased slightly due to unrelated packets
root@wrt3200acm:~# ifconfig lan1 | grep "RX bytes"
RX bytes:28856 (28.1 KiB) TX bytes:3062 (2.9 KiB)
root@wrt3200acm:~# ifconfig lan2 | grep "RX bytes"
RX bytes:3959 (3.8 KiB) TX bytes:7388 (7.2 KiB)
Routed scenario:
root@wrt3200acm:~# ip link del br0
root@wrt3200acm:~# ip link set lan1 up
root@wrt3200acm:~# ip addr add 10.255.1.1/30 dev lan1
root@wrt3200acm:~# ip link set lan2 up
root@wrt3200acm:~# ip addr add 10.255.1.5/30 dev lan2
root@wrt3200acm:~# iptables -I FORWARD -j ACCEPT
root@wrt3200acm:~# ifconfig lan1 | grep "RX bytes"
RX bytes:41466 (40.4 KiB) TX bytes:3524 (3.4 KiB)
root@wrt3200acm:~# ifconfig lan2 | grep "RX bytes"
RX bytes:11724 (11.4 KiB) TX bytes:7976 (7.7 KiB)
root@desktop:~# ip addr del 10.255.1.1/24 dev eth0
root@desktop:~# ip addr add 10.255.1.6/30 dev eth0
root@desktop:~# ip route add 10.255.1.0/30 via 10.255.1.5 dev eth0
root@desktop:~# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 128 KByte (default)
------------------------------------------------------------
[ 4] local 10.255.1.6 port 5001 connected with 10.255.1.2 port 53654
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 1.06 GBytes 907 Mbits/sec
[ 4] local 10.255.1.6 port 5001 connected with 10.255.1.2 port 53660
[ 4] 0.0-10.0 sec 1.06 GBytes 907 Mbits/sec
^Croot@desktop:~#
root@laptop:~# ip addr del 10.255.1.2/24 dev eth0
root@laptop:~# ip addr add 10.255.1.2/30 dev eth0
root@laptop:~# ip route add 10.255.1.4/30 via 10.255.1.1 dev eth0
root@laptop:~# iperf -c 10.255.1.6
------------------------------------------------------------
Client connecting to 10.255.1.6, TCP port 5001
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[ 3] local 10.255.1.2 port 53660 connected with 10.255.1.6 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 1.06 GBytes 908 Mbits/sec
root@laptop:~# iperf -c 10.255.1.6
------------------------------------------------------------
Client connecting to 10.255.1.6, TCP port 5001
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[ 3] local 10.255.1.2 port 53660 connected with 10.255.1.6 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 1.06 GBytes 908 Mbits/sec
root@laptop:~#
# Ifconfig counters increased by the amount of routed traffic
root@wrt3200acm:~# ifconfig lan1 | grep "RX bytes"
RX bytes:2351622094 (2.1 GiB) TX bytes:50939120 (48.5 MiB)
root@wrt3200acm:~# ifconfig lan2 | grep "RX bytes"
RX bytes:40160782 (38.2 MiB) TX bytes:2282781894 (2.1 GiB)
As can be seen, in a routed configuration (each port by itself, without a software bridge) the throughput is decreased and all forwarded frames are hitting the router CPU. In the bridged scenario, the iperf throughput is higher and almost all frames are bypassing the CPU.