RPi4b 13.0-Current, big performance hit with HDMI connected

[Andy McClements]

This may not be news to folks around here but it suprised me.

I was just doing some very quick performance checks having got my RPi4b 
to boot from SSD. I wanted to verify CPU frequency scaling was working 
so did a quick net and disk throughput check.

Any insights on this would be most welcome.

Results:

HDMI connected
==============

root at generic:~ # iperf3 -c 10.0.30.30
Connecting to host 10.0.30.30, port 5201
[  5] local 10.0.30.130 port 60995 connected to 10.0.30.30 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.01   sec  75.0 MBytes   625 Mbits/sec    0    209 KBytes
[  5]   1.01-2.00   sec  76.2 MBytes   641 Mbits/sec    0    209 KBytes
[  5]   2.00-3.00   sec  76.7 MBytes   645 Mbits/sec    0    209 KBytes
root at generic:~ # diskinfo -t /dev/da0
<snip>
Transfer rates:
         outside:       102400 kbytes in   0.694994 sec =   147339 
kbytes/sec
         middle:        102400 kbytes in   0.692708 sec =   147826 
kbytes/sec
         inside:        102400 kbytes in   0.692531 sec =   147863 
kbytes/sec

HDMI disconnected
=================

root at generic:~ # iperf3 -c 10.0.30.30
Connecting to host 10.0.30.30, port 5201
[  5] local 10.0.30.130 port 60384 connected to 10.0.30.30 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec   113 MBytes   946 Mbits/sec    0    208 KBytes
[  5]   1.00-2.00   sec   113 MBytes   947 Mbits/sec    0    208 KBytes
[  5]   2.00-3.00   sec   113 MBytes   947 Mbits/sec    0    208 KBytes
[  5]   3.00-4.00   sec   113 MBytes   947 Mbits/sec    0    208 KBytes


root at generic:~ # diskinfo -t /dev/da0
<snip>
Transfer rates:
         outside:       102400 kbytes in   0.541603 sec =   189068 
kbytes/sec
         middle:        102400 kbytes in   0.468219 sec =   218701 
kbytes/sec
         inside:        102400 kbytes in   0.473097 sec =   216446 
kbytes/sec