try (with-current-buffer (url-retrieve-synchronously "http://foo.bar" nil 't 1) (kill-buffer))

Documentation says:

(url-retrieve-synchronously URL &optional SILENT INHIBIT-COOKIES TIMEOUT)
...
"If TIMEOUT is passed, it should be a number that says (in seconds) how long to wait for a response before giving up."

Actually TIMEOUT is not working for me.

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GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)