Hi David,

In general the performance of big onemany fields has been a bit of a problem throughout history in Odoo. Odoo 11 and 12 have improved this as they use less calls (and better caching) between the frontend and backend causing lower loading times. The issue is still there though, it just got smaller. It is always a bit of a challenge to figure this out but as a general rule of thumbs you should try to minimalise the amount of computed fields and onchanges on models. The less you have of these the faster your loading will be.

Since Odoo V11 there is a great way to figure out performance issues by attaching a profiler to your functions. With this profiler you can see for every line of the function how long it takes so you'll know easily in which lines your performance problems are caused. See https://www.odoo.com/documentation/11.0/howtos/profilecode.html for more information. Sadly this is not available for V9/V10 but you could make a backport of the function in your Odoo in order to be able to measure it at your end.

I know this is not an ideal answer but atleast it gives you some pointers and ideas. You might also consider an upgrade to be able to combat part of the performance problems and having access to more tools. Even if you have no more computes and onchanges it will still need some time to load though, don't expect it to get under a second.

Regards,
Yenthe