系统自带的源实在太卡了。于是更换。
备份源
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bk
或者直接注释掉原来源（在前面加＃）
使用nano编辑器更改文件/etc/apt/sources.list
sudo nano /etc/apt/sources.list
#中科大
deb http://mirrors.ustc.edu.cn/raspbian/raspbian/ buster main contrib non-free rpi
#阿里云
#deb http://mirrors.aliyun.com/debian buster main
#deb http://mirrors.aliyun.com/debian-security buster/updates main
#deb http://mirrors.aliyun.com/debian buster-updates main
#清华
deb http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ buster main non-free contrib
deb-src http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ buster main non-free contrib按住ctrl＋w，再输入y，回车保存退出。下同
更新/etc/apt/sources.list.d/raspi.list
sudo nano /etc/apt/sources.list.d/raspi.list中科大：
deb http://mirrors.ustc.edu.cn/archive.raspberrypi.org/debian/ buster main最后更新一下源：
sudo apt update