SBS 2011 slow (network) access

After moving from SBS 2003 to SBS 2011 on new hardware, we experience annoying delays when opening files on network shares. For example, when double clicking a Word document to open it, it takes a couple of seconds before anything happens. We didn't experience this behaviour with the old server; a Pentium 4 3 GHz with 2 GB RAM and 1 Gbit network. The new server is a Dell PowerEdge T310 with a Xeon X3480, 16 GB RAM and also 1 Gbit network (Broadcom BCM5716C NetXtremeII). It has a PERC H700 RAID controller, accomodating two 600 GB SAS drives and two 2 TB near-line SAS drives, both in RAID 1. The switch we use is a Dell PowerConnect 5524.

The delay in opening files is very noticable with PDF files. We use Foxit reader version 4, which is placed on a network share. When opening a PDF file, it takes quite some time to load Foxit Reader and when loaded, it takes again quite some time to open the PDF file. When the stuff resided on the old SBS 2003 server, Foxit Reader and PDF's opened allmost instantly. And the same applies to other files...
 
I've been fiddling with the network settings and disabled QoS and 'Link Layer Topology' stuff, but that doesn't make a difference. Also looked at the network driversettings and all settings with 'offload' in their name are enabled. Switching off Norman NPRO (virusscanner) doesn't make a difference. I've been digging into document 'Performance Tuning Guidelines for Windows Server 2008 R2' (http://www.google.nl/url?sa=t&rct=j&q=Performance+Tuning+Guidelines+for+Windows+Server+2008+R2&source=web&cd=2&ved=0CDQQFjAB&url=http%3A%2F%2Fdownload.microsoft.com%2Fdownload%2F8%2FF%2F9%2F8F9CB30B-1617-4CF8-8534-9C2AAA0117F4%2FPerf-tun-srv-R2.docx&ei=q1zLTqecNoPsOdLx3YsP&usg=AFQjCNF1xSJwRcmMcfjL8APSOtxpf2mAzw&cad=rjt) and made some changes to network settings (page 22) like increasing the size of send/receive buffers. And on page 61 and 67, there are some registry settings to optimize the server / client for fileservices. I changed the registry entries to the recommended values, restarted SBS 2011 server and Win 7 Pro client, but there's no performance gain.

I downloaded and ran Ixia Qcheck to check the network speed. The TCP response time is less than 1 ms. The TCP throughput is about 80 MB / sec. I also ran a simple test: copy a file of about 32 GB from a client to the server and measure the time it takes. The outcome of this simple test resembles the throughput mentioned above.

Now this delay can have a multitude of causes, so I wonder how to find out the cause and how to solve it?

Simon