Well they decided to allow users to use arbitrary HTML themes for their blogs so there's not a whole lot they can do about that. The default theme isn't too terrible but a lot of the user themes are.
The worst is people using some old theme that embeds a script hosted on a site that no longer exists and the whole page load hangs until the request times out.
nah, even the default theme is very heavy and laggy...that's on the developers, not the users. Tumblr also uploads every GIF as an actual GIF, and doesn't convert them to webm or something that would reduce it to a fraction of the size. Lots of sites slow to a crawl because of this.
The worst is people using some old theme that embeds a script hosted on a site that no longer exists and the whole page load hangs until the request times out.