Describe the bug
Node has sync issue. Node isn't able to sync in normal speed. (They are all in local)

To Reproduce
Steps to reproduce the behavior:

1. Run a single CN and take another external node's P2P port into the single CN's protocol.json
2. Don't open the external node until CN's block height is more than 3000.
3. Start the external node, show state and watch sync performance.

Expected behavior
The external node will sync to about 500 then wait for several minutes than sync to about 1000 and so on.
It will take much time to sync to 3000 even there's no tx in each block.
Screenshots

Platform:

• OS: [Windows 10 x64]
• Version [latest master]

@ShawnYun