This doesn't seem like a bug, but it does leave me curious.

Just today I got a Raspberry Pi 4 set up with the 64-bit version of the OS formerly known as Raspbian. It runs off an SSD, and boots into the desktop in 18 seconds. Seems like it's running noticeably faster. So far, so good...

Arduino IDE 1.8.13 works fine, and compiles the default (empty) example for Uno without trouble.

When I install Teensyduino 1.53 and select Teensy 4.0 from the boards menu, it compiles and appears to link, but then it freezes at the very end:
Linking everything together...
(blah blah blah, bunch of stuff ending with arm-none-eabi-objdump, no errors)
/home/pi/arduino-1.8.13/hardware/teensy/../tools/teensy_post_compile -file=sketch_oct06a.ino -path=/tmp/arduino_build_571256 -tools=/home/pi/arduino-1.8.13/hardware/teensy/../tools/ -board=TEENSY40
That last line is where it hangs.

When I run strace on just that command, I see it's hanging while connecting to

Whoops... my firewall rule is only to accept related/established connections. I added a rule allowing traffic from localhost to anywhere, and it runs fine.

The question this raises: why is this thing opening a TCP connection to localhost? It doesn't seem harmful, just a little confusing.