I am experiencing speed problems during AP Job Related invoice line insertions on a very small site's workstations - each line on a single entry get slower and slower to process - by the 5th line this is already 15 seconds. Only one workstation does any real processing. The problem is still experienced when there are no other users (including non-Sage) on the network. Using RDP and running directly on the server is fine.

Server runs DB and App - Sage 300 2012 PU 3, Win 2012 Server R2, MS SQL Server 2008 Express R2, 8GB Ram, Xeon E5645 (2 processors) , 64bit.
Only 2 x Windows 7 Pro SP1 workstations, 4GB RAM, i3-2130 3.4GHz , 64bit. 
Neither workstation nor server resources maxing out during operations. 
100Mbit network - file copy based speed test is about 90,000,000 bit/s read or write but during Sage 300 processing nothing near this is reached.
Company database is just over 3GB.

Have already done: customization directories ok, PATH ok, reinstalled workstation, RegAcc re-run, set to Simple Logging, have temporarily turned all virus checkers off, UAC off, LAN speed-test (90mbit/s), ping 500x32 byte pings - no loss and all <1ms, nothing scary in any of the event logs, no strange services running on server or workstations, done dump and load and integrity. 

During installs I also only setup the connection in DB Setup and let the ODBC auto-create - on workstations this is SQL Server Native client 10.0 (v2009-100-1600-01). Am using the FQDN for shared/program folders/database "myserver.mydomain.local". and "myserver.mydomain.local\SQLEXPRESS"

Any recommendations or direction would be appreciated. Do I just need to give up with PJC and put full Sage 300 installs on the workstations?