Plugin Post Operation Lock record in Database

I am calling a web service from my plugin whih query the CRM databae and return some data to the plugin. When I register my plugin in Post-Operation Update I am getting a time out exception which is caused by the web service trying to query the databse. When I register my plugin in Pre-Operation Update it is working like a charm.

My Question is: DOES CRM DATABASE LOCK A RECORD ON POST OPERATION UPDATE ONLY AND NOT ON PRE OPERATION UPDATE?