Merging pieces I think definitely plays a part. Not merging when not needed, like 2 bridges or 2 corridors together would help. Just use the magnet feature but you have to sacrifice aesthetics a little.
Add line of sight blockers so that you cant see a bunch of peices. This worked rather well in Reach, so I assume it will have a similar affect here. there are also some peices (like glass) that create more framerate than others.
yeah the main issue I think for framerate is merging/phasing too many pieces, especially in cluttered areas. it's unfortunate because the best aesthetic designs are created by phasing objects together but you just have to do it in moderation and not go overboard.
Object density is a big deal and like Audience said, LoS blockers are a big help. The less objects you can see at a time the better. If you can use one piece where you have two, it'd help to cut it down substantially. Large intricate pieces that use a lot of polygons can cause FR. So limiting their use can help also.
I'm not so sure about the line of sight blockers. They only seem to help with texture pop-in. In my experience, once too many objects/polygons are in front of your view, regardless of whether they are blocked from sight, you'll experience frame-rate issues. I've made several maps where if you stand at one end and look toward the other, the frame rate begins to drop, particularly while strafing. It hasn't mattered whether I could see most of the objects or if they were blocked from view. On each of these maps, if you turn away from the majority of the map, the frame-rate is fine as would be expected. I do remember one thread from Reach where the posters seemed convinced that the coliseum walls acted as occluders, meaning that if a forge object was completely blocked from view by a coliseum wall, it wasn't rendered and the frame rate was improved. I've not tested it out though. Seems like it should be easy enough to test and confirm.
I have a map that I'll post a thread for later, which has some framerate problems on splitscreen. One area in particular is pretty bad. However, prior to this weekend it was somewhat laggy even in single player. I can tell you one thing for sure from my findings. WINDOWS. anything with windows is laggy. If you cram a few window-y pieces together tightly in a small area, then viewing that area along with eveyrthing else in the background will cause lag. Blocking line of sight was one of the ways I tested the area to find out which pieces could be removed with the biggest benefit. I had to come up with a less than ideal solution for the windows, but blocking them with a 4x4 flat piece did the trick temporarily for testing. As you said, strafing is when you'll notice framerate lag the most. I used the block piece to obstruct the view of several suspect portions of the view, and a cluster of windows was the biggest thing I fixed. I also removed some corridor 90s which I've heard are laggy if you use too many in an area. Interestingly enough, however, I have probably eight of these phased together as the ceiling (Lee, you would like how this building looks from the exterior, though you can't see it in-game intentionally) and there are no framerate problems while looking up at this roof. In fact, looking at it outside with most of the level viewable in single player also does not cause lag. I think distance has a huge part of it. So here is what I would stay away from for now... Clusters of complexity viewable from a relatively moderate or close distance. Use as few window-y pieces as possible. Likewise, spread out complexity all viewable from a moderate distance. Larger distances seem to reduce the texture quality as you get further away (mind you, this is farther than most Halo style levels usually get) so that lag is not as prevailant. In my experience thus far, phasing objects has no effect on framerate, other than the fact that two objects clustered together are more likely to appear on the screen simultaneously than two non-phased objects. Above all else, simply streamline and house-clean on your level. If you really look closely, chances are you'll find some areas where you are using two blocks when you could have used one, or even have built entirely around another piece that is no longer visible. Remove that piece! Or, perhaps a piece is only barely visible and if you make a small change it could be removed entirely. The plus side is this also frees up budget for you to use in less laggy areas.