Only calculate the maximum header size once and use ArrayPool to create the buffer.

src/ImageSharp/Bootstrapper.cs

@@ -38,6 +38,11 @@ namespace ImageSharp
         /// </summary>
         private readonly object syncRoot = new object();
 
+        /// <summary>
+        /// The maximum header size of all formats.
+        /// </summary>
+        private int maxHeaderSize;
+
         /// <summary>
         /// Prevents a default instance of the <see cref="Bootstrapper"/> class from being created.
         /// </summary>
@@ -50,6 +55,7 @@ namespace ImageSharp
                 new PngFormat(),
                 new GifFormat()
             };
+            this.SetMaxHeaderSize();
             this.parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount };
         }
 
@@ -63,6 +69,11 @@ namespace ImageSharp
         /// </summary>
         public static ParallelOptions ParallelOptions => Instance.parallelOptions;
 
+        /// <summary>
+        /// Gets the maximum header size of all formats.
+        /// </summary>
+        internal static int MaxHeaderSize => Instance.maxHeaderSize;
+
         /// <summary>
         /// Adds a new <see cref="IImageFormat"/> to the collection of supported image formats.
         /// </summary>
@@ -86,6 +97,8 @@ namespace ImageSharp
                 this.GuardDuplicate(format);
 
                 this.imageFormats.Add(format);
+
+                this.SetMaxHeaderSize();
             }
         }
 
@@ -114,5 +127,10 @@ namespace ImageSharp
                 }
             }
         }
+
+        private void SetMaxHeaderSize()
+        {
+            this.maxHeaderSize = imageFormats.Max(x => x.HeaderSize);
+        }
     }
 }

src/ImageSharp/Image/Image.cs

@@ -15,6 +15,7 @@ namespace ImageSharp
     using System.Threading.Tasks;
 
     using Formats;
+    using System.Buffers;
 
     /// <summary>
     /// Encapsulates an image, which consists of the pixel data for a graphics image and its attributes.
@@ -399,25 +400,34 @@ namespace ImageSharp
         /// </returns>
         private bool Decode(Stream stream)
         {
-            int maxHeaderSize = Bootstrapper.ImageFormats.Max(x => x.HeaderSize);
-            if (maxHeaderSize > 0)
+            int maxHeaderSize = Bootstrapper.MaxHeaderSize;
+            if (maxHeaderSize <= 0)
             {
-                byte[] header = new byte[maxHeaderSize];
+                return false;
+            }
 
-                stream.Position = 0;
+            IImageFormat format = null;
+            byte[] header = ArrayPool<byte>.Shared.Rent(maxHeaderSize);
+            try
+            {
+                long startPosition = stream.Position;
                 stream.Read(header, 0, maxHeaderSize);
                 stream.Position = 0;
+                format = Bootstrapper.ImageFormats.FirstOrDefault(x => x.IsSupportedFileFormat(header));
+            }
+            finally
+            {
+                ArrayPool<byte>.Shared.Return(header);
+            }
 
-                IImageFormat format = Bootstrapper.ImageFormats.FirstOrDefault(x => x.IsSupportedFileFormat(header));
-                if (format != null)
-                {
-                    format.Decoder.Decode(this, stream);
-                    this.CurrentImageFormat = format;
-                    return true;
-                }
+            if (format == null)
+            {
+                return false;
             }
 
-            return false;
+            format.Decoder.Decode(this, stream);
+            this.CurrentImageFormat = format;
+            return true;
         }
     }
 }