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
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
中科大：
deb http://mirrors.ustc.edu.cn/raspbian/raspbian/ buster main contrib non-free rpi
清华：
先安装https拉取： ｀ sudo apt install apt-transport-https｀
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释 deb https://mirrors.tuna.tsinghua.edu.cn/debian/ buster main contrib non-free # deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ buster main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-updates main contrib non-free # deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-updates main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-backports main contrib non-free # deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-backports main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian-security buster/updates main contrib non-free # deb-src https://mirrors.tuna.tsinghua.edu.cn/debian-security buster/updates main contrib non-free
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
４ 更新一下源：
sudo apt update