Doom3world

First off, I have checked the forums, but it's unusual circumstances as my map does not have doors. In the mod I am creating you teleport from room to room. I have a good understanding of VisPortals when using doors, but with this method of going from room to room I'm scratching my head as to how to place the VisPortals. Can someone point me in the right direction as to the best method, thanks in advance.

Visportals are meant to block visibility where the player is not meant to see (e.g. behind a door).
This helps the engine to decide not to render, or (for all intents and purposes, because there are more nuances) even think about, anything that lies behind the visportal until the door opens.
You place visportals in order to ensure performance for your maps.

Because your map goes from room to room you don't have to worry about doors and blocking things with visportals, so no worries there.
Placing visportals can be interesting though if your rooms are very detailed and have L shaped corridors or parts that can logically be blocked off by a visportal.

This explains visportals in a more detailed way: http://www.iddevnet.com/doom3/visportals.php

Unless your rooms are L shaped, have many corridors or are very detailed there's not much to worry about.
You can always check to see how much the engine is rendering by using the below console commands which will show all tris that are being rendered at that moment.

r_showTris 2
r_useScissor 0
r_showPortals 1

The r_useScissor 0 option is very important. As even though the areas aren't being rendered by the videocard, they are still being sent to the video card by the engine which is a very slow process.
So always test your visportals with r_useScissor set to 0. (Of course don't forget to set it back to 1 later on when you playtest your game since when you turn scissors off the data will not only be sent to your videocard it will also be rendered!).,

Interesting, thanks for your answer. I'm a little confused though as to why with each new room my map seems to get more jerky, when I look around it's smooth as silk, as soon as I start walking it's really jerky, both forward and back movements, that's why I thought it might be the lack of VisPortals.

Can you post a top overview of the map?

That's OK, I added a few VisPortals here and there and it's a little smoother. Thanks for your help, and congrats on Grimm, it's very immpressive, I only wish I had your skill in mapping and modding.

Keep on it - skill builds over time, only talent comes free and can easily get wasted.

Indeed, like BNA said, it's mostly hard work, resolve and just 'keeping at it'.