Tumblr may actually run more like a static site than dynamic as Bartholomew with Spin.
Tumblr may make use of CDN
We may have more cold start delay with Fermyon. It will definitely be way faster than standard serverless app, but there could be some cold start time over Tumblr.
On the other hand:
The newer blog will have way class CSS and javascript cruft. The Tumblr site is a mess for that.
We may also be able to use cloud-flare or something else to help with load time of static assets.
I suspect Tumblr will be faster because:
On the other hand:
We won't know until we have measured.