Transfer rate improves with Wireshark

I am trying to measure the read/write performance of a networked attached storage appliance using a Dell Vostro laptop/ Windows XP. Nominally, the read transfer rate is poor (ie read is worse than write). However, when I monitor the ethernet connection using WireShark (installed on the laptop), the read performance significantly improves.

Does Wireshark buffer data, or modify the transmission in any way that may explain this?