解決方案：https://askubuntu.com/questions/774986/appstreamcli-hanging-with-100-cpu-usage-during-update
搬運過來是：
This is caused by a bug https://bugs.launchpad.net/ubuntu/+source/appstream/+bug/1579712
The working solution (just tried myself):
First kill appstreamcli, either manually or with
sudo kill -KILL $(pgrep appstreamcli)
or
sudo pkill -KILL appstreamcli
Then:
wget -P /tmp https://launchpad.net/ubuntu/+archive/primary/+files/appstream_0.9.4-1ubuntu1_amd64.deb https://launchpad.net/ubuntu/+archive/primary/+files/libappstream3_0.9.4-1ubuntu1_amd64.deb
sudo dpkg -i /tmp/appstream_0.9.4-1ubuntu1_amd64.deb /tmp/libappstream3_0.9.4-1ubuntu1_amd64.deb
After this you will be able to proceed with apt-get update as usual