Hello, we are using this library to render a fairly large schema, and in our case it became quite painful to wait until it finished loading. I used Claude Opus to optimize it and ran a few review loops. It significantly decreased the loading time (1.5s vs 13s on my machine in the Chrome browser). All tests pass, and our large schema renders correctly. I hope these improvements will be useful to you.

Below Summary of changes from my fellow friend Opus-4.6

This PR reduces initial render time and schema loading overhead through several targeted optimizations.

Off-screen DOM build (core.js, editor.js)

The editor tree is now built while root_container is detached from the live DOM and re-attached in a single appendChild call. This eliminates per-element reflows during construction. A building flag suppresses onChange() calls for the duration. The build block is wrapped in try/finally to guarantee root_container is always re-attached and building is always cleared, even if a build step throws.
A new onContainerAttached() lifecycle hook (empty stub on AbstractEditor) is called on all editors after re-attachment, allowing editors to perform layout-dependent initialization that requires a live DOM.

Schema loading (schemaloader.js)

External ref loading is parallelized using Promise.all instead of sequential await per URI. The monolithic _asyncloadExternalRefs loop is split into _loadSingleRef, _loadUrnRef, and _loadAjaxRef helpers to improve readability and testability. onAllSchemasLoaded() is moved to the top-level load() method so it fires exactly once after the full ref graph resolves, restoring the pre-existing single-fire guarantee.
expandSchema() and expandRefs() results are memoized by schema object identity (Map) to avoid re-expanding the same schema repeatedly. The oneOf handler is fixed to use map() instead of reduce() and to avoid clobbering the parent schema's default when merging.

Validation error dispatch (object.js, array.js)

showValidationErrors() in both ObjectEditor and ArrayEditor now pre-buckets errors by direct child key/index before passing them down. Previously the full error array was passed to every child, causing O(n×m) path comparisons. Each child now only receives errors that belong to it.

MultipleEditor lazy validators (multiple.js)

Validator instances are now created lazily via _getValidator(i) instead of being eagerly instantiated for all types at build time. _getTypeSchema(i) is extracted as a shared helper used by both _getValidator and buildChildEditor. A getDefault() override checks per-type default values in oneOf/anyOf schemas before falling back to the parent default.

SignatureEditor off-screen fix (signature.js)

The canvas reference is stored on this.signatureCanvas and initial CSS sizing (width: 100%) is applied at build time. A onContainerAttached() override reads the real container width after the DOM is live and updates canvas.width accordingly, fixing the canvas sizing regression caused by off-screen construction.

Grid layout fix (object.js)

ObjectEditor adds an onContainerAttached() override that re-runs layoutEditors() for grid-format objects after the container is in the live DOM. The existing diff-check inside layoutEditors() makes the second call a no-op when heights do not affect the computed layout.

Misc

• extend({}, ...) replaced with { ...obj } or Object.assign throughout affected files
• Null-guard added to overwriteExistingProperties in utilities.js
• E2E test for issue Signature Pad not working #1367 gains a short wait to accommodate the deferred canvas resize