Combobox: typing gets quadratically slower with the number of rendered options (1000 options ≈ 3.5 s per keystroke) #3898

What package within Headless UI are you using?

@headlessui/react

What version of that package are you using?

v2.2.10 (React 19.3.0)

What browser are you using?

Chrome for Testing 153 (headless), arm64 Linux

Reproduction URL

https://github.com/corneliusroemer-agent/headlessui-combobox-many-options

Describe your issue

A non-virtual Combobox gets slow to type into as the number of rendered options grows: doubling the options makes a keystroke about 5× slower. Listbox with the same options stays fast.

Steps:

git clone https://github.com/corneliusroemer-agent/headlessui-combobox-many-options
cd headlessui-combobox-many-options
npm install && npm run build && npm run preview

1. Run above steps
2. Open http://localhost:4173/?n=1000
3. Click the input and type 1

The page shows the time from the input event to the next animation frame. ?c=listbox renders the same options in a static Listbox filtered by a plain <input> (?static=1 renders the Combobox options with static too, and is just as slow), and ?c=plain renders them as plain <li> elements.

Expected: updating 1000 options takes about as long as in Listbox (tens of ms), growing linearly with the option count.

Typing 1, median of 5 runs:

Typing 1 at n=1000 unmounts 729 options and mounts 729 new ones, and each ComboboxOption dispatches its own RegisterOption/UnregisterOption. In a CPU profile of that keystroke, about 55% of the time is shallowEqual under Machine.send, run for each of the three store subscriptions of every mounted option on every dispatch, and about 20% is compareDocumentPosition from re-sorting the option list. order={index} (?order=1) removes the sort but not the rest. Listbox batches registration into one dispatch; #3697 left the same change for Combobox to a follow-up.

Related:

• Improve Combobox component performance #3697, "Improve Combobox component performance", merged 2025-04-17 and shipped in 2.2.2. Its description says batching Combobox option registration, as Listbox and Menu already do, was left for a follow-up PR. I couldn't find that follow-up PR.
• Performance issues with ComboBox for big lists(100+ data) #2984, "Performance issues with ComboBox for big lists(100+ data)", February 2024, 4 comments, no maintainer answer. Mostly about the Vue package and predates the 2.2.2 rewrite, so it's related rather than the same bug.