I just have 2 SIM pre-paid & registered SIM cards (China Mobile network). One bought 2-3 years ago and one recently. I am a goon so just know they are GSM only. Never use in smartphones before.

Yes, I researched and understand one slot will be for 2G GSM network.

My question actually concerns the other slot (TD-SCDMA network). If I insert my other pre-paid SIM card inside, will I get the same 3G GSM network speed as if I am one of the China Mobile subscribers? I will be able to tap on to the wifi in my house, office and free public wifi?

Unfortunately, both China Mobile (& China Unicom) staff are unable to give me reliable info except words of assurance like the usual No Problem, Got Problem Come Back, Sure Can One...

Even if you bought an officially distributed China Mobile dual sim phone, like most (all?) dual sim phones out there the primary sim slot supports 3G while the secondary sim slot only supports 2G GSM.

This even applies to the high end dual sim phones out there including the dual sim versions of the Galaxy S4 and HTC One.

If you bought a cheap Chinese no-name dual sim phone, chances are both slots only support 2G GSM.

First we need to know what type of phone you are using?

A Fake Phone or clone? (Download speeds are slow on a clone phone)

An Android version? (Android OS can let you know that you are using Wifi for surfing rather then the 2Gor3G cards by looking at the top of the screen, you see icons telling you whats going on)

Next, Your prepaid cards are 2G or 3G?

2G boringly slow..

3G Fast enough to live with.