解决方案：更新为国内源

以中科大为例：

cd "$(brew --repo)"
git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/brew.git

cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/homebrew-core.git

brew update

第一次brew update的时候会较慢，后面使用就很快了。

如果不想使用国内源，可进行以下操作复原

cd "$(brew --repo)"
git remote set-url origin https://github.com/Homebrew/brew.git

cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
git remote set-url origin https://github.com/Homebrew/homebrew-core

brew update