[HELP] Unusual lag

[Sparky Sparky Boom Man]

Well, I say it's unusual because when I play Portal 2 in my notebook on my home, I didn't have any lag, but when I play it in my office (just like right now), I have a really heavy lag. Both with power adapter or using battery only, the lag comes.
Does a power source affect the game performance?
Help, I wanna play this in my office.

[Eetami]

I assume you're using the same computer?

My guess would be that
a) your office internet is too slow to play Portal 2 online, this might have various reasons OR
b) your office have blocked the required ports to play Portal 2 online, though this would prevent you from playing altogether OR
c) your offices router doesn't have necessary ports forwarded to play Portal 2, this causes HEAVY lag or disables you from playing altogether also.

This comes from assuming that
a) your "office" means your work office somewhere not in your house
b) "lag" in this case means internet lag.

If one or more of these aren't the case, say so, and I'll try to come up with something else.

Also no, power source does not affect game performance, provided you do have power.

[Sparky Sparky Boom Man]

Well, yes, I'm using the same computer. It's a notebook, actually.
I think the lag is not caused by internet connection, because I already tried to run Steam and play Portal 2 with Offline mode, but the lag still comes.

Oh, also, when I'm using my notebook at home, I'm using an external monitor with Extend mode (I set the external monitor as the main display so all the games will display in it) and the lag didn't comes, but when I'm using it at the office, of course I'm using the built-in monitor, and the lag comes when I'm using it.

And one more point, the lag is (probably) caused by my external wireless mouse. Because when I plug the receiver and Windows can read it, I got the lag. But when I unplug it, the lag will go. It only occurs with "built-in monitor at office" situation, it didn't occur when I played it at home.

I'm really not a smart person for fixing something like this. I don't know what to do.