I saw this issue that one of the US customers had when they tried to archive a large set of data.  During the archiving process, they get this error:

 "2025 Error Marshalling SourceCode.Workflow.Runtime.Management.WorkflowManagementHostServer.Archive, Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding."

This was related to the tables which stored the Activity XML fields (it was huge).

So there are a couple of options here.  You can try either option or both.

1. The slowness might be due to fragmented indexes on the tables.  You could try rebuilding the table indexes to boost the speed.  I have attached a simple SQL script which does this.  Just modify the database name in the script as required and run it.  Note that re-building the indexes takes some resources on the SQL server.  If the server is under load, it can impact performance and sometimes cause errors.  So it might be a good idea to run this during the off-hours.
2. You can increase the timeout setting in the SourceCode.Workflow.Runtime.Management.config file located in the <Program Files folder>\k2 blackpearl\Host Server\Bin directory.  Look for the ArchiveTimeout key and change its value.  It is set to 1000 seconds by default, which 16.67 minutes.  You can increase this value and try the archival again.