I'm looking for some suggestions, or feedback from anyone that has or has had this issue. I've about run out of things to try, and Google isn't helping me much either.

We run Windows 7 SP1, Office 2010. Both are up-to-date with patches.

The issue: Loading older (binary) format .xls files from our Sun 7110 storage takes between 5 and 10 minutes. Newer format xlsx files are pretty much instant.
The files I've tested aren't large - about 350k - and if I save them as xlsx or even xlsb in the same share/folder in the 7110 I can then open them almost instantly. They don't have lots of furmulae

Loading the same files locally from C: or from a newer NAS box (WD RX4100 which we've got running 2012R2) will load them pretty instantly.

I've looked at and discounted (through testing) GPOs, netsh tcp autotuninglevel, remote differential compression, local admin access

The Sun box and 2012 NAS both have LAN Manager compatibility level set at 2. This is for compatibility with other systems we have here.

Any suggestions (other than ditch the 7110)?