If you hang out in the Twine community you might recognise my alias, and that I have some experience regarding what can and can't be done with Twine.
Project's like this one and Secretary basically use Graphics in one of three ways:
- they display images and videos within the textual content of a Passage.
- they display images within Character / Show inventories.
- they layer images within a set area to implement "Paper Dolls"
And HTML5 has no issues with doing any of those things, and Twine just generates a HTML file that contains the project's:
- "Passage" meta-data
- Custom JavaScript
- and the selected Story Format's JavaScript based runtime engine.
The real cause of "slowdowns" is generally:
- the amount of stateful data the Author wants to store within each Moment of Progress History
- the maximum number of Moments the Author configures that Progress History to have.
- extensive looping through arrays of data.
- not optimising the size of the images and videos files being shown.
And unless you're intending to move the project to a compiled/native-level language, the above issues aren't going away.
But that's just my opinion...