The solution is as follows:
1. Click the wireless icon in the upper-right corner of the desktop, and then click Open Network Preferences in the pop-up menu.
2. In the network settings interface, click Advanced.
3. Click DNS in the window that opens.
4. Click the+button below the DNS server.
5. In the upper part, input the contents in turn, 8.8.8.8 and114.114, and finally click OK. If it still doesn't work, you can use Baidu DNS 180.
The above is the solution of slow download speed of bootcamp. Friends who encounter this problem can follow the above steps to solve it.