diff --git a/src/F23.StringSimilarity/F23.StringSimilarity.csproj b/src/F23.StringSimilarity/F23.StringSimilarity.csproj
index cd57f41..7115879 100644
--- a/src/F23.StringSimilarity/F23.StringSimilarity.csproj
+++ b/src/F23.StringSimilarity/F23.StringSimilarity.csproj
@@ -1,9 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
-  
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <PackageId>F23.StringSimilarity</PackageId>
-    <PackageVersion>5.0.0</PackageVersion>
     <PackageTags>string;similarity;distance;levenshtein;jaro-winkler;lcs;cosine</PackageTags>
     <Title>StringSimilarity.NET</Title>
     <Authors>James Blair, Paul Irwin</Authors>
@@ -18,18 +16,16 @@
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <IncludeSymbols>true</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+    <Version>5.1.0</Version>
   </PropertyGroup>
-
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
+    <PackageReference Include="System.Memory" Version="4.5.4" />
   </ItemGroup>
-
   <ItemGroup>
     <None Include="logo.png" Pack="true" PackagePath="\" />
   </ItemGroup>
-
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
     <DocumentationFile>bin\Release\netstandard2.0\F23.StringSimilarity.xml</DocumentationFile>
   </PropertyGroup>
-
 </Project>
\ No newline at end of file
diff --git a/src/F23.StringSimilarity/RatcliffObershelp.cs b/src/F23.StringSimilarity/RatcliffObershelp.cs
index 7dd2ba1..4a59af4 100644
--- a/src/F23.StringSimilarity/RatcliffObershelp.cs
+++ b/src/F23.StringSimilarity/RatcliffObershelp.cs
@@ -45,7 +45,7 @@ public double Similarity(string s1, string s2)
                 return 1.0d;
             }
 
-            var matches = GetMatchList(s1, s2);
+            var matches = GetMatchList(s1.AsSpan(), s2.AsSpan());
             int sumOfMatches = 0;
 
             foreach (var match in matches)
@@ -68,22 +68,22 @@ public double Distance(string s1, string s2)
             return 1.0d - Similarity(s1, s2);
         }
 
-        private static IList<string> GetMatchList(string s1, string s2)
+        private static IList<string> GetMatchList(ReadOnlySpan<char> s1, ReadOnlySpan<char> s2)
         {
             var list = new List<string>();
             var match = FrontMaxMatch(s1, s2);
 
             if (match.Length > 0)
             {
-                var frontSource = s1.Substring(0, s1.IndexOf(match, StringComparison.Ordinal));
-                var frontTarget = s2.Substring(0, s2.IndexOf(match, StringComparison.Ordinal));
+                var frontSource = s1.Slice(0, s1.IndexOf(match, StringComparison.Ordinal));
+                var frontTarget = s2.Slice(0, s2.IndexOf(match, StringComparison.Ordinal));
                 var frontQueue = GetMatchList(frontSource, frontTarget);
 
-                var endSource = s1.Substring(s1.IndexOf(match, StringComparison.Ordinal) + match.Length);
-                var endTarget = s2.Substring(s2.IndexOf(match, StringComparison.Ordinal) + match.Length);
+                var endSource = s1.Slice(s1.IndexOf(match, StringComparison.Ordinal) + match.Length);
+                var endTarget = s2.Slice(s2.IndexOf(match, StringComparison.Ordinal) + match.Length);
                 var endQueue = GetMatchList(endSource, endTarget);
 
-                list.Add(match);
+                list.Add(match.ToString());
                 list.AddRange(frontQueue);
                 list.AddRange(endQueue);
             }
@@ -91,17 +91,17 @@ private static IList<string> GetMatchList(string s1, string s2)
             return list;
         }
 
-        private static string FrontMaxMatch(string s1, string s2)
+        private static ReadOnlySpan<char> FrontMaxMatch(ReadOnlySpan<char> s1, ReadOnlySpan<char> s2)
         {
             int longest = 0;
-            var longestSubstring = "";
+            ReadOnlySpan<char> longestSubstring = ReadOnlySpan<char>.Empty;
 
             for (int i = 0; i < s1.Length; ++i)
             {
                 for (int j = i + 1; j <= s1.Length; ++j)
                 {
-                    var substring = s1.Substring(i, j - i);
-                    if (s2.Contains(substring) && substring.Length > longest)
+                    var substring = s1.Slice(i, j - i);
+                    if (s2.Contains(substring, StringComparison.Ordinal) && substring.Length > longest)
                     {
                         longest = substring.Length;
                         longestSubstring = substring;