I can't verify your numbers. As far as I can tell, loading a ~900,000 word document with no other differences than including or excluding </p> has about the same load time, though there's too much variance from load to load for me to really give definitive numbers.
Are you sure you converted it properly? I'd expect those kinds of numbers if your elements were very deeply nested by mistake (e.g. omitting tags where it's not valid to do so), but I don't see why leaving out </p> should be so slow.
For five runs, on the same hardware with the same load:
+ Unclosed: 4.00s, 3.91s, 3.59s, 4.45s, 3.93s
+ Closed: 3.90s, 2.74s, 3.9s, 2.05s, 3.39s
Though I'd note that the newline you have immediately following the paragraph, even when closing, would probably reduce the backtracking effect. And having no explicit body or head element would probably cause some different rendering patterns as well.
I don't know what you're measuring (onload?), but it's not giving you enough precision to make a conclusion about the performance of the HTML parser.
If you profile the page w/ devtools Performance panel, you'll see that just 5% of the CPU cost used to load & render the page is spent parsing the HTML. At that level I'm seeing costs of 22-36ms per load.
And, spoiler alert: after repeated runs I'm not seeing any substantial difference between these test pages. And based on how the HTML parser works, I wouldn't expect it.
Were the five unclosed runs before the five closed runs? I could see that making a difference vs. interleaving them, if the hardware needs to "warm up" first.
For me, on Firefox on Linux (I know it's the one with the smallest difference, but I don't have the others on hand, sorry), using the "load" time at the bottom of the Network tab, with cache disabled and refreshing with Ctrl+F5, interleaving the tests:
- Unclosed: 1.38s, 1.49s, 1.45s, 1.52s, 1.48s
- Closed: 1.47s, 1.37s, 1.48s, 1.49s, 1.35s
The one with </p> omitted takes about 0.032s longer on average going by these numbers, but that's about 2 frames of extra latency for a page almost twice the length of The Lord of the Rings.
Regarding the page itself, I tried to keep everything else as identical between the two versions as possible, including the DOM, hence why I wrote the </p> immediately before each <p>. As for backtracking, I'm not sure what you mean. The rule for the parser is simply "If the start tag is one from this list, and there's an open <p> element on the stack, close the <p> element before handling the start tag."
Are you sure you converted it properly? I'd expect those kinds of numbers if your elements were very deeply nested by mistake (e.g. omitting tags where it's not valid to do so), but I don't see why leaving out </p> should be so slow.
Try these two pages:
https://niconii.github.io/lorem-unclosed.html
https://niconii.github.io/lorem-closed.html