Slow Performance - Plugin for update Quotes

Hi all,

I have a Plugin that calls an external Web Service on activate quote button click. The process is this below:
1. The User clicks on Activate Quote button
2. Plugin calls the external web service and gets a return.
3. Then, i update one field of all the quotedetails with a new value returned by the web service.

This process have taken more then 5 minutes to complete, sometimes the user needs to wait 10 minutes. This time grows based on number of quotedetails in the Quote.

So, how can i reduce this time to finish that process?

Note: The plugin is running on real time and can't be assync.
Note: If the quote have 300 items, so the plugin will run 300 service.Update().