This happens sometimes and is not a cause for worry at all.
Here we have provided a detailed, step-by-step guide for troubleshooting: http://www.purevpn.com/speedup-tips.php We strongly recommend you try all techniques provided in above write-up. If none help then you will need to adjust your MTU settings in your Windows Registry.
Please follow these steps to modify the MTU settings:
First of all you will need to determine what is the optimal value of MTU for you: Look here: http://www.howtonetworking.com/VPN/mtu4.htm
Secondly, you will need to look on how to modify registry for correct settings, refer here: http://support.microsoft.com/kb/826159 For MAC, please refer here: http://support.apple.com/kb/HT2532 If this doesn't help, try to switch protocol ie PPTP to L2TP and vice versa. In 50% cases switching protocol solves the issue.