diff --git a/src/Type/TypeCombinator.php b/src/Type/TypeCombinator.php
index 838df16e51..6f36f1ceec 100644
--- a/src/Type/TypeCombinator.php
+++ b/src/Type/TypeCombinator.php
@@ -2084,7 +2084,21 @@ public static function doIntersect(Type ...$types): Type
 				$accessoryBaseTypes = null;
 				break;
 			}
-			$accessoryBaseTypes[] = $type->getDefaultBaseType();
+			// Accessory types share their default base type: every string accessory
+			// returns `string`, hasOffset() and hasOffsetValue() both return
+			// `array|ArrayAccess`. Adding the same base type again narrows nothing -
+			// intersection is idempotent - but the intersect() below distributes
+			// `A & (B | C)` one union at a time, so n copies of `array|ArrayAccess`
+			// would cost 2^n recursive calls before the duplicates are recognized at
+			// the leaves. That is why isset() with many offsets used to grow
+			// exponentially: each offset contributes one hasOffset().
+			$baseType = $type->getDefaultBaseType();
+			foreach ($accessoryBaseTypes as $addedBaseType) {
+				if ($addedBaseType->equals($baseType)) {
+					continue 2;
+				}
+			}
+			$accessoryBaseTypes[] = $baseType;
 		}
 		if ($accessoryBaseTypes !== null) {
 			// Accessory types never stand alone — supply the base type they refine.
diff --git a/tests/bench/data/bug-15061.php b/tests/bench/data/bug-15061.php
new file mode 100644
index 0000000000..86911e59e4
--- /dev/null
+++ b/tests/bench/data/bug-15061.php
@@ -0,0 +1,84 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15061;
+
+/**
+ * Every isset() subject narrows the array to hasOffset(), and intersecting n of them
+ * used to cost 2^n TypeCombinator::intersect() calls: each hasOffset() contributes the
+ * same `array|ArrayAccess` default base type, and those n identical unions were
+ * distributed over each other one at a time.
+ *
+ * @phpstan-type FooEntity array{
+ *     a?: string,
+ *     b?: string,
+ *     c?: string,
+ *     d?: string,
+ *     e?: string,
+ *     f?: string,
+ *     g?: string,
+ *     h?: string,
+ *     i?: string,
+ *     j?: string,
+ *     k?: string,
+ *     l?: string,
+ *     m?: string,
+ *     n?: string,
+ *     o?: string,
+ *     p?: string,
+ *     q?: string,
+ *     r?: string,
+ *     s?: string,
+ *     t?: string,
+ *     u?: string,
+ *     v?: string,
+ *     w?: string,
+ *     x?: string,
+ *     y?: string,
+ *     z?: string,
+ * }
+ */
+final class TestClass
+{
+
+	public function __invoke(): void
+	{
+		/** @var array<string, FooEntity> $entities */
+		$entities = [];
+
+		foreach ($entities as $entity) {
+			$ok = isset(
+				$entity['a'],
+				$entity['b'],
+				$entity['c'],
+				$entity['d'],
+				$entity['e'],
+				$entity['f'],
+				$entity['g'],
+				$entity['h'],
+				$entity['i'],
+				$entity['j'],
+				$entity['k'],
+				$entity['l'],
+				$entity['m'],
+				$entity['n'],
+				$entity['o'],
+				$entity['p'],
+				$entity['q'],
+				$entity['r'],
+				$entity['s'],
+				$entity['t'],
+				$entity['u'],
+				$entity['v'],
+				$entity['w'],
+				$entity['x'],
+				$entity['y'],
+				$entity['z'],
+			);
+
+			if (!$ok) {
+				continue;
+			}
+		}
+	}
+
+}