This is already marked as resolved question, but I want to share I discovered. I found that HTTP request’s fetch rate is controlled by FHttpModule, especially HttpThreadActiveFrameTimeInSeconds variable is the key. Default value of it is around 0.005(200Hz). That means, tiny fetch operations were called just 200 times per second. By decreasing the value to the proper value I could get reasonable download speed.
You can control this value from the Engine configuration file like this:
[HTTP]
HttpThreadActiveFrameTimeInSeconds=0.00001