diff --git a/package.json b/package.json
index 9d27aec7f..a612623e1 100644
--- a/package.json
+++ b/package.json
@@ -43,7 +43,7 @@
     "cheminfo-types": "^1.15.0",
     "fft.js": "^4.0.4",
     "is-any-array": "^3.0.0",
-    "ml-matrix": "^6.12.2",
+    "ml-matrix": "^6.13.0",
     "ml-xsadd": "^3.0.1"
   },
   "devDependencies": {
diff --git a/src/utils/fftCache.ts b/src/utils/fftCache.ts
index acc4ff401..e81887ddd 100644
--- a/src/utils/fftCache.ts
+++ b/src/utils/fftCache.ts
@@ -9,7 +9,7 @@ class FFTCache {
   #maxSize: number;
   readonly #instances = new Map<number, FFT>();
 
-  constructor(maxSize = 10) {
+  constructor(maxSize = 1) {
     this.#maxSize = maxSize;
   }
 
@@ -77,7 +77,7 @@ export function clearFFTCache(): void {
 
 /**
  * Sets the maximum number of distinct transform sizes kept in the shared FFT
- * cache (default 10). When the cache is full and a new size is requested, the
+ * cache (default 1). When the cache is full and a new size is requested, the
  * whole cache is dropped and rebuilt on demand. Lowering the limit below the
  * current number of cached sizes clears the cache immediately.
  * @param maxSize - maximum number of cached sizes; must be a positive integer.
diff --git a/src/xyArray/utils/getSlotsToFirst.ts b/src/xyArray/utils/getSlotsToFirst.ts
index a0441bc66..cdd2a450c 100644
--- a/src/xyArray/utils/getSlotsToFirst.ts
+++ b/src/xyArray/utils/getSlotsToFirst.ts
@@ -20,6 +20,7 @@ export interface Slot {
  * Computes slots aligned to the first spectrum's x values, adding slots for any x values not covered.
  * @param data - data.
  * @param options - options.
+ * @returns the slots, sorted by x value, with non-overlapping `[from, to)` ranges.
  */
 export function getSlotsToFirst(
   data: DataXY[],
@@ -42,10 +43,15 @@ export function getSlotsToFirst(
 
   const otherXs = xyArrayWeightedMerge(data.slice(1), options).x;
   let currentPosition = 0;
-  for (const slot of slots) {
+  // Only the first-spectrum slots drive this pass; slots created for uncovered
+  // x values are appended and merged in by the sort below, so we freeze the
+  // count instead of iterating an array we are mutating.
+  const firstSlotCount = slots.length;
+  for (let i = 0; i < firstSlotCount; i++) {
+    const slot = slots[i];
     while (
-      otherXs[currentPosition] < slot.to &&
-      currentPosition < otherXs.length
+      currentPosition < otherXs.length &&
+      otherXs[currentPosition] < slot.to
     ) {
       if (otherXs[currentPosition] < slot.from) {
         const currentDelta = deltaIsFunction
diff --git a/src/xyArray/xyArrayAlignToFirst.ts b/src/xyArray/xyArrayAlignToFirst.ts
index fd3572446..b79a4efc4 100644
--- a/src/xyArray/xyArrayAlignToFirst.ts
+++ b/src/xyArray/xyArrayAlignToFirst.ts
@@ -16,6 +16,7 @@ export interface XYArrayAlignToFirstOptions {
  * If some x values are missing in the first spectrum we will add them
  * @param data - data
  * @param options - options
+ * @returns the common `x` axis and one aligned `y` array per input spectrum.
  */
 export function xyArrayAlignToFirst(
   data: DataXY[],
@@ -26,7 +27,10 @@ export function xyArrayAlignToFirst(
 } {
   const { delta = 1 } = options;
   const slots = getSlotsToFirst(data, { delta });
-  const x = Float64Array.from(slots.map((slot) => slot.value));
+  const x = new Float64Array(slots.length);
+  for (let i = 0; i < slots.length; i++) {
+    x[i] = slots[i].value;
+  }
   const ys = Array.from(data, () => new Float64Array(x.length));
 
   const positions = new Uint32Array(data.length);