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Yes, I meant 301, server-side redirects. Regarding performance, I currently have a little over 50,000 entries in my redirects file with no discernable performance impact. But, different platforms handle differently, and also we have a CDN which caches redirects too, so that could make a difference. I guess the safest approach would be to insert a hundred thousand or more dummy redirect entries into your redirect file, temporarily, and stress test it.