diff --git a/src/coverlet.core/Coverage.cs b/src/coverlet.core/Coverage.cs
index e6750cefa..59439a91d 100644
--- a/src/coverlet.core/Coverage.cs
+++ b/src/coverlet.core/Coverage.cs
@@ -161,6 +161,8 @@ private void CalculateCoverage()
                     continue;
                 }
 
+                List<Document> documents = result.Documents.Values.ToList();
+
                 using (var fs = new FileStream(result.HitsFilePath, FileMode.Open))
                 using (var sr = new StreamReader(fs))
                 {
@@ -173,7 +175,7 @@ private void CalculateCoverage()
                             continue;
 
                         bool isBranch = info[0] == "B";
-                        var document = result.Documents.ElementAt(int.Parse(info[1])).Value;
+                        var document = documents[int.Parse(info[1])];
 
                         int start = int.Parse(info[2]);
                         int hits = int.Parse(info[4]);
diff --git a/src/coverlet.core/Instrumentation/Instrumenter.cs b/src/coverlet.core/Instrumentation/Instrumenter.cs
index 5925298e9..c89c42d75 100644
--- a/src/coverlet.core/Instrumentation/Instrumenter.cs
+++ b/src/coverlet.core/Instrumentation/Instrumenter.cs
@@ -175,17 +175,12 @@ private void InstrumentIL(MethodDefinition method)
 
         private Instruction AddInstrumentationCode(MethodDefinition method, ILProcessor processor, Instruction instruction, SequencePoint sequencePoint)
         {
-            int documentIndex = 0;
             if (!_result.Documents.TryGetValue(sequencePoint.Document.Url, out var document))
             {
                 document = new Document { Path = sequencePoint.Document.Url };
-                documentIndex = _result.Documents.Count;
+                document.Index = _result.Documents.Count;
                 _result.Documents.Add(document.Path, document);
             }
-            else
-            {
-                documentIndex = _result.Documents.Keys.ToList().IndexOf(document.Path);
-            }
 
             for (int i = sequencePoint.StartLine; i <= sequencePoint.EndLine; i++)
             {
@@ -193,7 +188,7 @@ private Instruction AddInstrumentationCode(MethodDefinition method, ILProcessor
                     document.Lines.Add(i, new Line { Number = i, Class = method.DeclaringType.FullName, Method = method.FullName });
             }
 
-            string marker = $"L,{documentIndex},{sequencePoint.StartLine},{sequencePoint.EndLine}";
+            string marker = $"L,{document.Index},{sequencePoint.StartLine},{sequencePoint.EndLine}";
 
             var pathInstr = Instruction.Create(OpCodes.Ldstr, _result.HitsFilePath);
             var markInstr = Instruction.Create(OpCodes.Ldstr, marker);
@@ -208,17 +203,12 @@ private Instruction AddInstrumentationCode(MethodDefinition method, ILProcessor
 
         private Instruction AddInstrumentationCode(MethodDefinition method, ILProcessor processor, Instruction instruction, BranchPoint branchPoint)
         {
-            int documentIndex = 0;
             if (!_result.Documents.TryGetValue(branchPoint.Document, out var document))
             {
                 document = new Document { Path = branchPoint.Document };
-                documentIndex = _result.Documents.Count;
+                document.Index = _result.Documents.Count;
                 _result.Documents.Add(document.Path, document);
             }
-            else
-            {
-                documentIndex = _result.Documents.Keys.ToList().IndexOf(document.Path);
-            }
 
             var key = (branchPoint.StartLine, (int)branchPoint.Ordinal);
             if (!document.Branches.ContainsKey(key))
@@ -235,7 +225,7 @@ private Instruction AddInstrumentationCode(MethodDefinition method, ILProcessor
                     }
                 );
 
-            string marker = $"B,{documentIndex},{branchPoint.StartLine},{branchPoint.Ordinal}";
+            string marker = $"B,{document.Index},{branchPoint.StartLine},{branchPoint.Ordinal}";
 
             var pathInstr = Instruction.Create(OpCodes.Ldstr, _result.HitsFilePath);
             var markInstr = Instruction.Create(OpCodes.Ldstr, marker);
diff --git a/src/coverlet.core/Instrumentation/InstrumenterResult.cs b/src/coverlet.core/Instrumentation/InstrumenterResult.cs
index 615e2e316..dcd82694a 100644
--- a/src/coverlet.core/Instrumentation/InstrumenterResult.cs
+++ b/src/coverlet.core/Instrumentation/InstrumenterResult.cs
@@ -27,6 +27,7 @@ public Document()
         }
 
         public string Path;
+        public int Index;
 
         public Dictionary<int, Line> Lines { get; private set; }
         public Dictionary<(int Line, int Ordinal), Branch> Branches { get; private set; }
diff --git a/src/coverlet.tracker/CoverageTracker.cs b/src/coverlet.tracker/CoverageTracker.cs
index c41accff5..6486a2f68 100644
--- a/src/coverlet.tracker/CoverageTracker.cs
+++ b/src/coverlet.tracker/CoverageTracker.cs
@@ -8,6 +8,7 @@ namespace Coverlet.Tracker
     public static class CoverageTracker
     {
         private static List<Dictionary<string, Dictionary<string, int>>> _events;
+        private static readonly StringHashSuffixComparer _stringHashSuffixComparer = new StringHashSuffixComparer();
 
         [ThreadStatic]
         private static Dictionary<string, Dictionary<string, int>> t_events;
@@ -25,7 +26,7 @@ public static void MarkExecuted(string file, string evt)
         {
             if (t_events == null)
             {
-                t_events = new Dictionary<string, Dictionary<string, int>>();
+                t_events = new Dictionary<string, Dictionary<string, int>>(_stringHashSuffixComparer);
                 lock (_events)
                 {
                     _events.Add(t_events);
@@ -38,7 +39,7 @@ public static void MarkExecuted(string file, string evt)
             {
                 if (!t_events.TryGetValue(file, out var fileEvents))
                 {
-                    fileEvents = new Dictionary<string, int>();
+                    fileEvents = new Dictionary<string, int>(_stringHashSuffixComparer);
                     t_events.Add(file, fileEvents);
                 }
 
diff --git a/src/coverlet.tracker/StringHashSuffixComparer.cs b/src/coverlet.tracker/StringHashSuffixComparer.cs
new file mode 100644
index 000000000..78f0cd0fd
--- /dev/null
+++ b/src/coverlet.tracker/StringHashSuffixComparer.cs
@@ -0,0 +1,35 @@
+using System.Collections.Generic;
+
+namespace Coverlet.Tracker
+{
+    internal class StringHashSuffixComparer : IEqualityComparer<string>
+    {
+        public bool Equals(string x, string y) => string.Equals(x, y);
+
+        public int GetHashCode(string s)
+        {
+            if (s == null || s.Length == 0)
+                return 0;
+
+            // Hash calculation based on the old implementation of NameTable used in System.Xml
+            const int SuffixLength = 8;
+            const int Seed = 1031880390;
+            int hashCode;
+            unchecked
+            {
+                hashCode = s.Length + Seed;
+                int i = s.Length > SuffixLength ? s.Length - SuffixLength : 0;
+                for (; i<s.Length; ++i)
+                {
+                    hashCode += (hashCode << 7) ^ s[i];
+                }
+
+                hashCode -= hashCode >> 17;
+                hashCode -= hashCode >> 11;
+                hashCode -= hashCode >> 5;
+            }
+
+            return hashCode;
+        }
+    }
+}