Django ModelForm：如何很少执行昂贵的现场验证？

I have a model with about a dozen fields. Its instances are created via a form with also about a dozen fields. One of the fields is a URL for which I need to check actual accessibility. That check requires a login and takes considerable time (about 4 seconds), so I want to perform it as rarely as possible -- but also as early as a user would expect, namely, right after the URL was entered.I have a model with about a dozen fields. Its i