diff --git a/src/browser/http.ts b/src/browser/http.ts
index 5e586d5..0ca8baa 100644
--- a/src/browser/http.ts
+++ b/src/browser/http.ts
@@ -14,6 +14,8 @@ export interface Pack {
   offsets: Map<string, { image: string; at: number }>;
 }
 
+const utf8 = new TextDecoder();
+
 // Pack layout: repeated [u16 path length][path][u32 size][bytes]. In the boot
 // pack, "/index.prefix" holds prefix-coded "d path" and "f path size
 // [image:offset] [pack]" lines. A file with an offset is read in ranges from
@@ -24,11 +26,10 @@ export interface Pack {
 export function parsePack(buf: ArrayBuffer): Pack {
   const dv = new DataView(buf);
   const u8 = new Uint8Array(buf);
-  const dec = new TextDecoder();
   const files = new Map<string, Uint8Array>();
   for (let p = 0; p < buf.byteLength;) {
     const n = dv.getUint16(p);
-    const path = dec.decode(u8.subarray(p + 2, p + 2 + n));
+    const path = utf8.decode(u8.subarray(p + 2, p + 2 + n));
     p += 2 + n;
     const size = dv.getUint32(p);
     files.set(path, u8.subarray(p + 4, p + 4 + size));
@@ -38,7 +39,7 @@ export function parsePack(buf: ArrayBuffer): Pack {
   const raw = files.get("/index.prefix") ?? files.get("/index.txt");
   if (raw) {
     pack.index = new PackIndex(pack);
-    addIndex(pack, dec.decode(raw), files.has("/index.prefix"));
+    addIndex(pack, utf8.decode(raw), files.has("/index.prefix"));
     files.delete("/index.txt");
     files.delete("/index.prefix");
     for (const [p, data] of files) {
@@ -56,6 +57,9 @@ export function parsePack(buf: ArrayBuffer): Pack {
 export class PackIndex extends Map<string, number> {
   readonly modules = new Map<string, Uint8Array>();
   private readonly pack: Pack;
+  // directory -> names in it, built for the entries there were at the time
+  private dirs: Map<string, string[]> | null = null;
+  private dirsSize = 0;
 
   constructor(pack: Pack) {
     super([["/", -1]]);
@@ -72,6 +76,24 @@ export class PackIndex extends Map<string, number> {
     return super.has(path);
   }
 
+  // The names in each directory. Entries are only ever added, so a change in
+  // size means new ones.
+  children(): Map<string, string[]> {
+    if (this.dirs && this.dirsSize === this.size) return this.dirs;
+    const dirs = new Map<string, string[]>();
+    for (const p of this.keys()) {
+      if (p === "/") continue;
+      const i = p.lastIndexOf("/");
+      const parent = i === 0 ? "/" : p.slice(0, i);
+      let names = dirs.get(parent);
+      if (!names) dirs.set(parent, (names = []));
+      names.push(p.slice(i + 1));
+    }
+    this.dirs = dirs;
+    this.dirsSize = this.size;
+    return dirs;
+  }
+
   // Parses the lines of the module path is in. The module directories
   // themselves are in the main index.
   load(path: string) {
@@ -82,7 +104,7 @@ export class PackIndex extends Map<string, number> {
     const raw = this.modules.get(module);
     if (!raw) return;
     this.modules.delete(module);
-    addIndex(this.pack, new TextDecoder().decode(raw), true);
+    addIndex(this.pack, utf8.decode(raw), true);
   }
 }
 
@@ -297,9 +319,5 @@ export function listIndex(idx: PackIndex, path: string): string[] | null {
   if (idx.get(path) !== -1) return null;
   const prefix = path === "/" ? "/" : path + "/";
   idx.load(prefix);
-  const out: string[] = [];
-  for (const p of idx.keys()) {
-    if (p.startsWith(prefix) && p.length > prefix.length && !p.slice(prefix.length).includes("/")) out.push(p.slice(prefix.length));
-  }
-  return out;
+  return [...(idx.children().get(path) ?? [])];
 }
diff --git a/src/jvm.ts b/src/jvm.ts
index add6487..6b2f6c3 100644
--- a/src/jvm.ts
+++ b/src/jvm.ts
@@ -117,7 +117,8 @@ export class JVM {
   private natives = new Map<string, NativeFn>();
   private interned = new Map<string, JObject>();
   readonly prims = new Map<string, JClass>();
-  private slots = new Map<string, number>();
+  // owner -> field name -> slot; two lookups of literal strings beat building a key
+  private slots = new Map<string, Map<string, number>>();
   readonly threads = new Set<JThread>();
   private runQueue: JThread[] = [];
   private queued = new Set<JThread>();
@@ -716,14 +717,14 @@ export class JVM {
   // ---- fields by name ------------------------------------------------------
 
   slotOf(owner: string, name: string): number {
-    const key = owner + "." + name;
-    let s = this.slots.get(key);
+    let byName = this.slots.get(owner);
+    if (!byName) this.slots.set(owner, (byName = new Map()));
+    let s = byName.get(name);
     if (s === undefined) {
-      const c = this.loadBootOrFail(owner);
-      const f = c.findField(name);
-      if (!f) throw new Error(`no field ${key}`);
+      const f = this.loadBootOrFail(owner).findField(name);
+      if (!f) throw new Error(`no field ${owner}.${name}`);
       s = f.slot;
-      this.slots.set(key, s);
+      byName.set(name, s);
     }
     return s;
   }
diff --git a/src/natives/nio.ts b/src/natives/nio.ts
index 5771d09..df7336c 100644
--- a/src/natives/nio.ts
+++ b/src/natives/nio.ts
@@ -88,10 +88,8 @@ function fillStat(t: JThread, attrs: JObject, st: Stat, writable: boolean, ino:
   vm.setField(attrs, A, "st_uid", 1000);
   vm.setField(attrs, A, "st_gid", 1000);
   vm.setField(attrs, A, "st_size", BigInt(size));
-  for (const k of ["atime", "mtime", "ctime"]) {
-    vm.setField(attrs, A, `st_${k}_sec`, sec);
-    vm.setField(attrs, A, `st_${k}_nsec`, nsec);
-  }
+  for (const k of ["st_atime_sec", "st_mtime_sec", "st_ctime_sec"]) vm.setField(attrs, A, k, sec);
+  for (const k of ["st_atime_nsec", "st_mtime_nsec", "st_ctime_nsec"]) vm.setField(attrs, A, k, nsec);
 }
 
 // open mount table handles: whether the one entry has been read