Hmmmmm.
Either that specific test (I didn't make it, to clarify) or Chromium has a bug.
Depending on how I fiddle with the browser, the test either takes 134 seconds or 6 (!!!) seconds.
Of course when I ran the test for the first time I did it a few times. I think the repeated tests may have triggered these bugs.
If I go to the test page, click the (i) to the left of the domain, hit Cookies, select the domain (the root of the tree listview) and Remove (to remove the IDB, WebSQL and localStorage bits the page sets up) and then run the test, I consistently get 6 seconds.
My suspicion is that, the page is creating 1000 items in localStorage, THEN adding my 10000 on top of that. That would explain the crazy delays.
However, this does not explain why, when I visited the page for the first time in several months, it completed in 6 seconds without any tinkering required.
Note that the page re-fills localStorage on each load - clearing it after the page has loaded is key.