diff --git a/src/string.cr b/src/string.cr
index d3bc7d6998b2..60205fe547c6 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -3312,15 +3312,20 @@ class String
   # Update rolling hash for Rabin-Karp algorithm `String#index`.
   private macro update_hash(n)
     {% for i in 1..n %}
-      {% if i != 1 %}
-        byte = head_pointer.value
-      {% end %}
+      byte = head_pointer.value
       hash = hash &* PRIME_RK &+ pointer.value &- pow &* byte
       pointer += 1
       head_pointer += 1
     {% end %}
   end
 
+  private macro update_simplehash(n)
+    {% for i in 1..n %}
+      hash = (hash << 8) | pointer.value
+      pointer += 1
+    {% end %}
+  end
+
   # Returns the index of the _first_ occurrence of *search* in the string, or `nil` if not present.
   # If *offset* is present, it defines the position to start the search.
   #
@@ -3354,23 +3359,45 @@ class String
     nil
   end
 
+  private def index_short(hash_type, offset : Int32, pointer : UInt8*, end_pointer : UInt8*, search, &)
+    search_hash = hash_type.new(0)
+    hash = hash_type.new(0)
+    mask = hash_type.new(0)
+
+    search.each_byte do |b|
+      search_hash = (search_hash << 8) | b
+      hash = (hash << 8) | pointer.value
+      mask = (mask << 8) | 0xff
+      pointer += 1
+    end
+
+    while true
+      return offset if (hash & mask) == search_hash
+
+      char_bytesize = yield pointer
+      return if pointer + char_bytesize > end_pointer
+      case char_bytesize
+      when 1 then update_simplehash 1
+      when 2 then update_simplehash 2
+      when 3 then update_simplehash 3
+      else        update_simplehash 4
+      end
+
+      offset &+= 1
+    end
+  end
+
   # :ditto:
   def index(search : String, offset = 0)
     offset += size if offset < 0
     return if offset < 0
 
     return size < offset ? nil : offset if search.empty?
+    return index(search[0], offset) if search.size == 1 && search.valid_encoding?
 
     # Rabin-Karp algorithm
     # https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm
 
-    # calculate a rolling hash of search text (needle)
-    search_hash = 0u32
-    search.each_byte do |b|
-      search_hash = search_hash &* PRIME_RK &+ b
-    end
-    pow = PRIME_RK &** search.bytesize
-
     # Find start index with offset
     char_index = 0
     pointer = to_unsafe
@@ -3381,16 +3408,26 @@ class String
       char_index += 1
     end
 
-    head_pointer = pointer
+    return if pointer + search.bytesize > end_pointer
 
-    # calculate a rolling hash of this text (haystack)
+    if search.bytesize <= 8
+      search_bytesize = search.bytesize
+      return index_short(UInt64, char_index, pointer, end_pointer, search) do |pointer|
+        String.char_bytesize_at(pointer - search_bytesize)
+      end
+    end
+
+    head_pointer = pointer
+    search_hash = 0u32
     hash = 0u32
-    hash_end_pointer = pointer + search.bytesize
-    return if hash_end_pointer > end_pointer
-    while pointer < hash_end_pointer
+
+    # calculate a rolling hash of search text (needle) and this text (haystack)
+    search.each_byte do |b|
+      search_hash = search_hash &* PRIME_RK &+ b
       hash = hash &* PRIME_RK &+ pointer.value
       pointer += 1
     end
+    pow = PRIME_RK &** search.bytesize
 
     while true
       # check hash equality and real string equality
@@ -3398,7 +3435,6 @@ class String
         return char_index
       end
 
-      byte = head_pointer.value
       char_bytesize = String.char_bytesize_at(head_pointer)
       return if pointer + char_bytesize > end_pointer
       case char_bytesize
@@ -3685,12 +3721,7 @@ class String
     offset += bytesize if offset < 0
     return if offset < 0
 
-    offset.upto(bytesize - 1) do |i|
-      if to_unsafe[i] == byte
-        return i
-      end
-    end
-    nil
+    to_slice.fast_index(byte.to_u8, offset)
   end
 
   # Returns the index of the _first_ occurrence of *char* in the string, or `nil` if not present.
@@ -3708,32 +3739,16 @@ class String
   # "Dizzy Miss Lizzy".byte_index('z', -4)  # => 13
   # "Dizzy Miss Lizzy".byte_index('z', -17) # => nil
   # ```
-  def byte_index(char : Char, offset = 0) : Int32?
-    return byte_index(char.ord, offset) if char.ascii?
+  def byte_index(char search : Char, offset = 0) : Int32?
+    return byte_index(search.ord, offset) if search.ascii?
 
     offset += bytesize if offset < 0
     return if offset < 0
-    return if offset + char.bytesize > bytesize
-
-    # Simplified "Rabin-Karp" algorithm
-    search_hash = 0u32
-    search_mask = 0u32
-    hash = 0u32
-    char.each_byte do |byte|
-      search_hash = (search_hash << 8) | byte
-      search_mask = (search_mask << 8) | 0xff
-      hash = (hash << 8) | to_unsafe[offset]
-      offset += 1
-    end
+    return if offset + search.bytesize > bytesize
 
-    offset.upto(bytesize) do |i|
-      if (hash & search_mask) == search_hash
-        return i - char.bytesize
-      end
-      # rely on zero terminating byte
-      hash = (hash << 8) | to_unsafe[i]
-    end
-    nil
+    pointer = to_unsafe + offset
+    end_pointer = to_unsafe + bytesize
+    index_short(UInt32, offset, pointer, end_pointer, search) { 1 }
   end
 
   # Returns the byte index of *search* in the string, or `nil` if the string is not present.
@@ -3754,27 +3769,31 @@ class String
     return if offset < 0
 
     return bytesize < offset ? nil : offset if search.empty?
+    return byte_index(search.to_unsafe[0], offset) if search.bytesize == 1
 
     # Rabin-Karp algorithm
     # https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm
 
-    # calculate a rolling hash of search text (needle)
+    pointer = to_unsafe + offset
+    end_pointer = to_unsafe + bytesize
+    return if pointer + search.bytesize > end_pointer
+
     search_hash = 0u32
-    search.each_byte do |b|
-      search_hash = search_hash &* PRIME_RK &+ b
+    hash = 0u32
+
+    if search.bytesize <= 8
+      return index_short(UInt64, offset, pointer, end_pointer, search) { 1 }
     end
-    pow = PRIME_RK &** search.bytesize
 
-    # calculate a rolling hash of this text (haystack)
-    pointer = head_pointer = to_unsafe + offset
-    hash_end_pointer = pointer + search.bytesize
-    end_pointer = to_unsafe + bytesize
-    hash = 0u32
-    return if hash_end_pointer > end_pointer
-    while pointer < hash_end_pointer
+    head_pointer = pointer
+
+    # calculate a rolling hash of search text (needle) and this text (haystack)
+    search.each_byte do |b|
+      search_hash = search_hash &* PRIME_RK &+ b
       hash = hash &* PRIME_RK &+ pointer.value
       pointer += 1
     end
+    pow = PRIME_RK &** search.bytesize
 
     while true
       # check hash equality and real string equality
@@ -3785,9 +3804,7 @@ class String
       return if pointer >= end_pointer
 
       # update a rolling hash of this text (haystack)
-      hash = hash &* PRIME_RK &+ pointer.value &- pow &* head_pointer.value
-      pointer += 1
-      head_pointer += 1
+      update_hash 1
       offset += 1
     end