If a complex constituent ad-hoc query with tens of thousands of results and many result fields containing revenue fields and relationship fields is running, then a user tries to merge an organization, performance degrades significantly due to sustained blocking.
We are currently evaluating this issue for a fix in a future service pack.
Steps to Duplicate
1) Create a constituent query filtering on every organization. In the result fields, use a dozen or so Relationships\Related Constituent fields, including Relationships\Related Constituent\Revenue fields and Relationships\Related Constituent\Revenue\ Education (Primary). 2) Go to the Preview results tab, which should take several minutes. 3) A minute or two after the query starts running, open a second tab in CRM. 4) In the new tab, go to an organization's record > Merge organization > Select a secondary organization and the other required fields > Save 5) While the merge and query are running simultaneously, notice that navigation around the environment is slow and some users may have issues logging in to the environment.