Why is it so slow to decompress System. IO. Compression. GZipStream?

Writing the following code makes me very laborious.
. Net provides the Compression example of System. IO. Compression. GZipStream, but there is no good code for decompression.
However, after extracting a section and compressing it, it becomes 1.16 MB, while the original data is 15 MB,
It took up to 3000 milliseconds!

I don't know why. Please kindly advise me!

First paste the Code:

 1    1. public object DeCompression(byte[] request)  
 2    2. {  
 3    3.     object ob = new object();  
 4    4.     Stream sourceStream = new MemoryStream(request);  
 5    5.     System.IO.Compression.GZipStream zipStream = new System.IO.Compression.GZipStream(  
 6    6.         sourceStream,  
 7    7.         System.IO.Compression.CompressionMode.Decompress);  
 8    8.     byte[] buffer = new byte[20480];  
 9    9.     byte[] bt = new byte[0];  
10   10.     int iread = 0;  
11   11.     do  
12   12.     {  
13   13.         iread = zipStream.Read(buffer, 0, 20480);  
14   14.         byte[] bswap = new byte[bt.Length + buffer.Length];  
15   15.         Buffer.BlockCopy(bt, 0, bswap, 0, bt.Length);  
16   16.         Buffer.BlockCopy(buffer, 0, bswap, bt.Length, buffer.Length);  
17   17.         bt = bswap;  
18   18.         bswap = null;  
19   19.         GC.Collect();  
20   20.     } while (iread > 0);  
21   21.     zipStream.Close();  
22   22.     sourceStream.Close();  
23   23.     MemoryStream ms = new MemoryStream(bt);  
24   24.     ob = DeSerializeBinary(ms);  
25   25.     ms.Close();  
26   26.     return ob;  
27   27. }