diff --git a/src/Init/Data/String/Defs.lean b/src/Init/Data/String/Defs.lean
index b9d5fa5acb..f864e3d2c4 100644
--- a/src/Init/Data/String/Defs.lean
+++ b/src/Init/Data/String/Defs.lean
@@ -685,7 +685,20 @@ abbrev endValidPos (s : String) : s.Pos :=
   s.endPos
 
 @[deprecated String.toByteArray (since := "2025-11-24")]
-abbrev String.bytes (s : String) : ByteArray :=
+abbrev bytes (s : String) : ByteArray :=
   s.toByteArray
 
+/--
+Returns the length of the string `s`, assuming the string is comprised only of ASCII characters.
+
+This is implemented as a synonym for `s.utf8ByteSize`, which takes constant time.
+-/
+@[inline]
+def lengthAssumingAscii (s : String) : Nat :=
+  s.utf8ByteSize
+
+@[simp]
+theorem lengthAssumingAscii_eq {s : String} : s.lengthAssumingAscii = s.utf8ByteSize :=
+  (rfl)
+
 end String
diff --git a/src/Init/System/Uri.lean b/src/Init/System/Uri.lean
index 033252f293..5e7e5ae046 100644
--- a/src/Init/System/Uri.lean
+++ b/src/Init/System/Uri.lean
@@ -14,6 +14,7 @@ import Init.Omega
 import Init.System.Platform
 import Init.While
 import Init.Data.String.Length
+import Init.Data.Iterators.Combinators.Take
 
 public section
 
@@ -93,35 +94,42 @@ a single unicode code point and these will also be decoded correctly. -/
 def unescapeUri (s: String) : String :=
   UriEscape.decodeUri s
 
+private def normalizeDriveLetter (uri : String) : String :=
+  -- lower-case drive letters seem to be preferred in URIs
+  match (uri.chars.take 2).toList with
+  | [driveLetter, ':'] => if driveLetter.isUpper then uri.decapitalize else uri
+  | _ => uri
+
 /-- Convert the given FilePath to a "file:///encodedpath" Uri. -/
 def pathToUri (fname : System.FilePath) : String := Id.run do
   let mut uri := fname.normalize.toString
   if System.Platform.isWindows then
-    -- normalize drive letter
-    -- lower-case drive letters seem to be preferred in URIs
-    if uri.length >= 2 && uri.front.isUpper && String.Pos.Raw.get uri ⟨1⟩ == ':' then
-      uri := uri.decapitalize
+    uri := normalizeDriveLetter uri
     uri := uri.map (fun c => if c == '\\' then '/' else c)
   uri := uri.foldl (fun s c => s ++ UriEscape.uriEscapeAsciiChar c) ""
   let result := if uri.startsWith "/" then "file://" ++ uri else "file:///" ++ uri
   result
 
+-- On Windows, the path "/c:/temp" needs to become "C:/temp"
+private def normalizeDriveExpression (p : String.Slice) : String :=
+  match (p.chars.take 3).toList with
+  | ['/', driveLetter, ':'] =>
+    if driveLetter.isAlpha then
+      (p.drop 1).copy.capitalize
+    else p.copy
+  | _ => p.copy
+
 /-- Convert the given uri to a FilePath stripping the 'file://' prefix,
 ignoring the optional host name. -/
-def fileUriToPath? (uri : String) : Option System.FilePath := Id.run do
-  if !uri.startsWith "file://" then
-    none
-  else
-    let mut p := (unescapeUri uri).drop "file://".length |>.copy
-    p := p.dropWhile (λ c => c != '/') |>.copy -- drop the hostname.
+def fileUriToPath? (uri : String) : Option System.FilePath :=
+  match (unescapeUri uri).dropPrefix? "file://" with
+  | none => none
+  | some p =>
+    let p := p.dropWhile (fun c => c != '/') -- drop the hostname.
     if System.Platform.isWindows then
-      -- On Windows, the path "/c:/temp" needs to become "C:/temp"
-      if p.length >= 2 &&
-          p.front == '/' && (String.Pos.Raw.get p ⟨1⟩).isAlpha && String.Pos.Raw.get p ⟨2⟩ == ':' then
-        -- see also `pathToUri`
-        p := String.Pos.Raw.modify (p.drop 1).copy 0 .toUpper
-      p := p.map (fun c => if c == '/' then '\\' else c)
-    some p
+      some ((normalizeDriveExpression p).map (fun c => if c == '/' then '\\' else c))
+    else
+      some p.copy
 
 end Uri
 end System
diff --git a/src/lake/Lake/CLI/Main.lean b/src/lake/Lake/CLI/Main.lean
index e724e00340..6a30236c32 100644
--- a/src/lake/Lake/CLI/Main.lean
+++ b/src/lake/Lake/CLI/Main.lean
@@ -228,9 +228,9 @@ def validateRepo? (repo : String) : Option String := Id.run do
     return "invalid characters in repository name"
   match repo.split '/' |>.toStringList with
   | [owner, name] =>
-    if owner.length > 39 then
+    if owner.lengthAssumingAscii > 39 then
       return "invalid repository name; owner must be at most 390 characters long"
-    if name.length > 100 then
+    if name.lengthAssumingAscii > 100 then
       return "invalid repository name; owner must be at most 100 characters long"
   | _ => return "invalid repository name; must contain exactly one '/'"
   return none
@@ -271,7 +271,7 @@ def lakeLongOption : (opt : String) → CliM PUnit
   modifyThe LakeOptions ({· with scope? := some (.ofRepo repo)})
 | "--platform" => do
   let platform ← takeOptArg "--platform" "cache platform"
-  if platform.length > 100 then
+  if platform.chars.length > 100 then
     error "invalid platform; platform is expected to be at most 100 characters long"
   modifyThe LakeOptions ({· with platform? := some <| .ofString platform})
 | "--toolchain" => do
diff --git a/src/lake/Lake/CLI/Shake.lean b/src/lake/Lake/CLI/Shake.lean
index 0174245ca9..88672ba695 100644
--- a/src/lake/Lake/CLI/Shake.lean
+++ b/src/lake/Lake/CLI/Shake.lean
@@ -326,7 +326,7 @@ where
   addExplanation (j : ModuleIdx) (k : NeedsKind) (use def_ : Name) (deps : Explanations) : Explanations :=
     if
       if let some (some (name', _)) := deps[(j, k)]? then
-        decide (use.toString.length < name'.toString.length)
+        decide (use.toString.chars.length < name'.toString.chars.length)
       else true
     then
       deps.insert (j, k) (use, def_)
diff --git a/src/lake/Lake/Config/Cache.lean b/src/lake/Lake/Config/Cache.lean
index 208bca7d47..1fe4afa5be 100644
--- a/src/lake/Lake/Config/Cache.lean
+++ b/src/lake/Lake/Config/Cache.lean
@@ -497,7 +497,7 @@ public def system : CachePlatform := ⟨System.Platform.target⟩
 @[inline] public def ofString (s : String) : CachePlatform := ⟨s⟩
 
 /-- Returns the length of the platform identifier in Unicode code points. -/
-public def length (self : CachePlatform) : Nat := self.raw.length
+public def length (self : CachePlatform) : Nat := self.raw.chars.length
 
 /-- Returns a string representation of the platform identifier. -/
 public protected def toString (self : CachePlatform) : String :=
@@ -529,7 +529,7 @@ public def ofString (s : String) : CacheToolchain := ⟨normalizeToolchain s⟩
 @[inline] public def ofElanToolchain (s : String) : CacheToolchain := ⟨s⟩
 
 /-- Returns the length of the toolchain identifier in Unicode code points. -/
-public def length (self : CacheToolchain) : Nat := self.raw.length
+public def length (self : CacheToolchain) : Nat := self.raw.chars.length
 
 /-- Returns a string representation of the toolchain identifier. -/
 public protected def toString (self : CacheToolchain) : String :=