To avoid holding up everything else when deleting massive amounts of data from an SQL Server table, you can use the TOP() function to delete the data in chunks.
SELECT 1 WHILE @@ROWCOUNT > 0 BEGIN DELETE TOP(1000) FROM [dbo].[mytable] WHERE timestampUtc < '2015-05-29' END
The “SELECT 1” primes the @@ROWCOUNT variable, so the while loop will run. Yes, there are indeed prettier ways of doing this, but it doesn’t really matter.