Update README.md

Author: wkh237

README.md

@@ -728,7 +728,9 @@ Here's a [sample app](https://github.com/wkh237/rn-firebase-storage-upload-sampl
 
 **Reduce RCT Bridge and BASE64 Overheard**
 
-React Native connects JS and Native context by passing JSON through React bridge, therefore there will be an overhead to convert data before they sent. When data is large, this will be quite a performance impact to your app, it's recommended to use file storage instead of BASE64 if possible. The following chart shows how much faster when loading data from storage than BASE64 encoded string on iphone 6.
+React Native connects JS and Native context by passing JSON around React Native bridge, and there will be an overhead to convert data before they sent to each side. When data is large, this will be quite a performance impact to your app, it's recommended to use file storage instead of BASE64 if possible. 
+
+There's a knowning issue when reading data via `fs.readStream`. the process seems blocking JS thread when file is large, it's because the default buffer size is quite small (4kb) which result in large amount of events triggered in JS thread, try to increase the buffer size (for example 100kb => 102400) to reduce the frequency. The following chart shows how much faster when loading data from storage than BASE64 encoded string on iphone 6.
 
 <img src="img/performance_1.png" style="width : 100%"/>