It takes 3m45s to load 30k products here with Odoo 8.0

It does not matter if the PC running the till app is our old touchscreen POS (Win XP 1GB ram and some legacy Celeron processor), or my i7 laptop with 16GB of RAM.

The loading time oscilates around the same mark.

I too am a bit disappointed in this, as it virtually stops us from adding more products. I know I need to chuck in about 100k products for good measure, and I am worried that at some stage I might start experiencing timeouts.

Anyone has any ideas?

Your only option to improve the POS speed would probably be to clean up the code yourself or to upgrade your Odoo from V7 -> V8. The speeds have increased since V7.. Another option (which you probably have already tried) is to add better RAM, CPU, ... and upgrade all your hardware specs but I'm quite sure you've tried that and that isn't ideal..