So, to me, Tumblr's website is already the main point of access, and these performance problems don't exist on Firefox or Chrome. I'm not talking about server-side response times, I'm talking about the time to render posts on the client. I find that a lot of times, after scrolling, you have to wait a few seconds before you see anything but the blue background that Tumblr has.
So, no, I'm going to pin it on Safari if (even) Firefox can deal with it.
Yahoo runs one of the biggest ad networks in the world, and you need to register to use Tumblr. They as already have everything they need to track you right there.
A mobile app in many circumstances reduces the data tracking (see this whole discussion about how effective Apple's do not track is because of their monopoly powers).
Just because Apple made it harder doesn't mean they made it impossible. There's a reason so many places work on making apps for multiple platforms (at least 2) rather than a unified web experience. There are benefits beyond serving a webpage in a native app, and they all want those benefits. Stick your head in the sand and deny it all you want, but it still happens.
However, curiosity requires that I ask what/how/why does any of that affect mobile-first web deployment in away that it is not addressed when a large chunk of that mobile use is broken? If you program yourself into a dead end, back up and take another turn.
Oh, it is easier in a mobile native where you get the benefit of hoovering up personal data on all of your users? Gee, let's not expend effort to make something work universally, let's instead take the easy route and make money on the side too. The fact that losing this large share of users because of one type of content is not enough of a decision to go the other route shows just how much money there is in the hoovering of data.
Still putting the blame on Tumblr.