diff --git a/src/helpers.ts b/src/helpers.ts
index b692f030..7bebbcc5 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -9,33 +9,81 @@ import { type AnyNode, hasChildren, type ParentNode } from "domhandler";
  * @returns Remaining nodes that aren't contained by other nodes.
  */
 export function removeSubsets(nodes: AnyNode[]): AnyNode[] {
-    let index = nodes.length;
+    const { length } = nodes;
 
     /*
-     * Check if each node (or one of its ancestors) is already contained in the
-     * array.
+     * A lone node is the most common input by far, and the only member it can
+     * be contained by is itself, so walk its ancestors directly rather than
+     * building a set for one entry.
      */
-    while (--index >= 0) {
+    if (length < 2) {
+        if (length === 1) {
+            const node = nodes[0];
+            for (
+                let ancestor = node.parent;
+                ancestor;
+                ancestor = ancestor.parent
+            ) {
+                if (ancestor === node) {
+                    nodes.length = 0;
+                    break;
+                }
+            }
+        }
+        return nodes;
+    }
+
+    /*
+     * Membership is only ever tested against the nodes that were passed in, so
+     * collect them up front. That replaces the `includes` scan run for every
+     * ancestor of every node, which made this O(n^2 * depth), with an O(1)
+     * lookup per ancestor. Each slot is read once and kept, since a getter on
+     * the array would otherwise be invoked twice.
+     */
+    const members = new Set<AnyNode>();
+    const slots: AnyNode[] = [];
+    for (let index = 0; index < length; index++) {
         const node = nodes[index];
+        slots.push(node);
+        members.add(node);
+    }
 
-        /*
-         * Remove the node if it is not unique.
-         * We are going through the array from the end, so we only
-         * have to check nodes that preceed the node under consideration in the array.
-         */
-        if (index > 0 && nodes.lastIndexOf(node, index - 1) >= 0) {
-            nodes.splice(index, 1);
-            continue;
+    /*
+     * A second set is only needed to drop repeats. If every node was distinct
+     * there are none, so the common case allocates just the one set.
+     */
+    const seen = members.size === length ? null : new Set<AnyNode>();
+    let kept = 0;
+
+    for (let index = 0; index < length; index++) {
+        const node = slots[index];
+
+        /* Keep the first occurrence of each node, as `lastIndexOf` did. */
+        if (seen !== null) {
+            if (seen.has(node)) continue;
+            seen.add(node);
         }
 
+        let contained = false;
         for (let ancestor = node.parent; ancestor; ancestor = ancestor.parent) {
-            if (nodes.includes(ancestor)) {
-                nodes.splice(index, 1);
+            if (members.has(ancestor)) {
+                contained = true;
                 break;
             }
         }
+
+        if (contained) continue;
+
+        /*
+         * Compact the survivors in place, as the splices did. Only write when
+         * the value actually moves, so that an array needing no changes is
+         * never written to -- a frozen array survived the original untouched.
+         */
+        if (kept !== index) nodes[kept] = node;
+        kept++;
     }
 
+    if (kept !== length) nodes.length = kept;
     return nodes;
 }
 /**
@@ -135,9 +183,36 @@ export function compareDocumentPosition(
  * @returns Collection of unique nodes, sorted in document order.
  */
 export function uniqueSort<T extends AnyNode>(nodes: T[]): T[] {
-    nodes = nodes.filter(
-        (node, index, array) => !array.includes(node, index + 1),
-    );
+    /* Nothing to dedupe or sort, and no reason to allocate a Set. */
+    if (nodes.length < 2) {
+        return nodes;
+    }
+
+    /*
+     * Keep the LAST occurrence of each node, as `array.includes(node, index + 1)`
+     * did. That matters for nodes in different documents: they compare equal, and
+     * the sort below is stable, so which duplicate survives decides the output
+     * order. Walking backwards and reversing preserves that, while making the
+     * dedupe O(n) instead of O(n^2).
+     */
+    const seen = new Set<T>();
+    const unique: T[] = [];
+    for (let index = nodes.length - 1; index >= 0; index--) {
+        /*
+         * `filter` skipped holes in a sparse array, so an absent slot must not
+         * become an `undefined` entry in the result.
+         */
+        if (!(index in nodes)) {
+            continue;
+        }
+        const node = nodes[index];
+        if (!seen.has(node)) {
+            seen.add(node);
+            unique.push(node);
+        }
+    }
+    unique.reverse();
+    nodes = unique;
 
     nodes.sort((a, b) => {
         const relative = compareDocumentPosition(a, b);