Progressive slowing of NIS GUI launch

This is going to be a lengthy post so apologies in advance.

Since updating to NIS v21 last year I have had a problem not seen on earlier versions in that launching the admin GUI, which is very rapid on initial installation, takes progressively longer as time passes.

I got the automatic update from v20 to v21 last June.  Initially all was fine but by late September the launch time for the GUI was getting very extended.  A secondary problem then appeared in that the Norton Product Maintenance task started running for 30-40 minutes rather than the usual 1-2 seconds.  I was going to try uninstall/reinstall but before I could do that I suffered a hard drive failure.  With a warranty replacement drive from Dell, I did a fresh installation of Windows and my applications software. I then downloaded a fresh installation of NIS v21 and, once again, initially the GUI launch time was pretty much instantaneous.  That was in November but it progressively slowed and by early February it was taking in excess of a minute.  Then the extended run times of Product Maintenance started happening again.  I decided to uninstall/reinstall again and tried to uninstall but the Uninstall process froze at the "Preparing to Uninstall" stage.  I killed that and had to use the Norton Removal Tool instead to get rid of the existing NIS installation.

I downloaded and reinstalled NIS again and, yet again, the GUI launch time was initially rapid.  Now at the start of April, only two months later, it's become very slow again.  Currently, when I launch the GUI (either from the system tray icon, the desktop shortcut or the Start menu), the Norton icon appears in the Task bar immediately but it's around 15 seconds before the GUI window appears on the screen, and a further 15-20 seconds for the window to be populated with the icons and CPU usage stats.  The display then freezes for around another 30 seconds before the links/buttons become active.  Throughout the delay there is continuous hard drive traffic.  Once the GUI is launched and active, there are no delays in moving between the different options - it's just the initial launch that is slow.  (This time round I haven't yet seen the long Product Maintenance run times.)

I'm currently running NIS 21.7.0.11 plus the Norton extensions for Chrome (Toolbar 2014.7.10.28 and Identity Safe 1.0.5).  The rest of my setup is:

Dell Optiplex 790 with Windows 7 Pro 64-bit,  Microsoft Office 2010 Pro ( I only use Word, Excel and Outlook), Google Picassa 3, Google Chrome 41.0.2272.101m with the two Norton extensions and Adblock extension, APC Powerchute (UPS monitoring), Canoscan Toolbox 4.9 (Canon scanner utilities), Adobe Reader XI and Seagate Manager (data backup utility).

I've tried regularly exporting and clearing the Norton history logs to minimise any log file overhead.  I also discovered that the batches of blocked file access reports that appear frequently in the Tamper Protection history are due to Norton blocking the Windows defrag service so I have tried turning Tamper Protection off to allow the Norton files to be be defragmented.  I've also run the Norton Optimiser defragger regularly.  None of this has improved things.

Any suggestions or solutions?  Am I alone in seeing this problem?