It is definitely the asset specs that caused your problem, and with that number no timeout adjustments will help. We had the same situation, with far fewer records, recently when specs were loaded and Mobile suddenly could not cope with it. It is supporting data so your work order association will not work. If you are only using the Assets app then you probably could tweak the object structure query based on Custodian. However, as I understand it from our situation, the asset specs are also in the supporting data for Techncian, SRs, and Inspections so all would have to be modified.
You do not mention your Maximo/Manage version, but I understand there are improvements in this area inlcuding right up to the latest fix pack out any day now.
My advice would be to open a Case with support so you will get advive directly from the source, along with fixes or workarounds specific to your scenario.