CPU will never get to 100% as it's a multi-core cpu and Inventor will only ever use 1 core (except for things like FEA, Moldflow and Rendering). So a quad-core chip at 25% is running at full pelt for Inventor. If your 6-core chip registers a 17% load with Inventor then it's at full capacity and being the bottleneck.
basically, unless you're doing lots of rendering or FEA then an expensive Xeon chip isn't needed and you'll get the same (if not better) performance out of a sandy/ivy-bridge i5.
What other applications are running if you're seeing over 17% load?
MemProbe - goto Process and choose Inventor to monitor (instead of all applications). It will now show how much physical and virtual memory Inventor is using:
don't forget to keep all drivers up to date - not just gpu but also chipset (motherboard). The chipset is like the backbone of the system, if it's not running as quickly/well as it should then everything isn't going to work at their capacity. Sounds crazy, but old mouse drivers have caused problems in the past...
Finally - make sure your virus-scanner is setup to NOT scan Inventor files (add .ipt/.iam/etc to the exclusion list) - if it's scanning every single part when you load and access an assembly it can slow it all down...
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