I just loaded a link to a tweet in a private browsing window on my iPhone in some sub-second amount of time. So whatever issue you're running into is a bug, not intentional.
Interesting how different our experiences are. This is a two-month old iphone, fully patched and with very few apps installed. Same experience as my previous iphone and android phones. Just verified: I sat and watched the "loading" spinner for over three minutes before starting to write this response. No idea how long it actually took, but it finished with just a blank white page with a twitter logo and a search bar. Maybe it's my mobile network (AT&T), who knows. But the experience is basically unusable, and I see at least one other person in this thread mentioning the error that indicates requests are being throttled, so I know I'm not alone.
I get frequent failures for various reasons on multiple connection types, including Google Fiber. Referrer seems to be part of it. Mobile does seem much worse (WiFi or cell network) which makes it feel intentional.