I'm getting a lot of lag when I activate too many shells in one minecraft session: I take it this can be avoided by not using too many separate shells in one session? Or is this a bug or is this a natural result of having too many shells on a map?

Notes: SSP, world frequently opened to LAN to play with housemates, though this was not an issue until I actually built my shells, numerous shells to be explained below.
Stronghold (1 shell in storage, 0 in constructor)
Base (1, 0)
Mushroom 1 (1, 1)
Mushroom 2 (0, 1)
Mushroom 3 (0, 1)
Mushroom 4 (1, 1)

So, predominantly jumping between those 4 stored shells (as I was concerned stored shells contributed more to RAM consumption than ones waiting in shell constructors) and ignoring (when possible) the four in the constructors.
Co-ordinates on these loci can be provided on request.

Stronghold should be 1,1... I forgot about the one in the constructor there... Totally forgot about that one. So, 4 stored shells between which I jump and 5 waiting in constructors.

Instead of writing comments, you can edit your original post if you need to make a correction or something.

Each shell acts as a chunk loader for the chunk it's in. What are your computer specs like and which version of Sync are you using? What other mods do you have and how "advanced" are you in this world? What is the average TPS with all these active? I need more details. Does this only happen when truely single player or when open to LAN or both? Did you actually find it less laggy when using ones in constructors compared to stored ones?

Ok, let me get into answering all these.

1. I'll do my best to do the edit thing in future unless replying. Sorry if you got a zerg rush of notifications.
2. So, this means that having one shell in the constructor AND one in the shell storage right next to it will have 0 influence on the game as they're both in the same chunk. Right?
3. The latest version, to my understanding. I only upgraded to 1.6.4 a week ago so that I could install Sync and Morph. I'm really just checking iChun's site for the number, as I'm away from my computer until tomorrow night. I downloaded the latest version when I did so. 2.2.1 is listed as the latest version, and that rings a bell.
4. Computer:
   12GB RAM
   Don't know what TPS is.
   It's actually mostly been with single player, but I do less shell jumping in LAN as I've purposefully kept the sync mod a secret from my housemates. :P
5. Mods running:
   Haribote airships
   Ore dimensions (which is currently acting very strangely and messing up some portals and having some items not exist... I wonder if that's being interfered with by one of iChun's mods)
   Morph
   Sync mod (congrats on winning ModJam with that, by the way)
   Utility Chests (just introduces some new types of chest into the game)
   Treecapitator (drop entire tree with bottom block destroyed)
   BspkrsCore (spelling? Inactive prerequisite mod for treecapitator)
   Rotten Flesh to Leather (allows smelting of rotten flesh into leather)
   Rei's Minimap (adds a mini map to the top right corner of the screen)
   TooMuchTNT

I had the Iron Man Armors mod, but it conflicted with morph mod's ability to let me fly as a mob.

I haven't encountered any difference in legginess between keeping them in the constructor vs. storage, I'd just figured that they wouldn't exist yet until I took them out of the constructor. If this is not how it should work, I'll adjust for that.
6. Finally, here's some rough co-ordinates for the sake of understanding how much of my world I'm keeping loaded.
Stronghold: 1130, 225, 25 (1s, 1c)
Base: 600, 30 (1s, 0c)
Mushroom 1: -3040, -78, 60 (1s, 1c)
Mushroom 2: -4800, -285, 80 (0s, 1c)
Mushroom 3: -8700, -470, 45 (0s, 1c)
Mushroom 4: -12400, -750, 40 (1s, 1c)
I consider myself pretty advanced at this point. Not much complex redstone going on, but a few dozen portals spread across the world, lots of mining... a few mine cart railroads in the Nether... I haven't been to The End yet, but that and giant complicated redstone inventions are pretty much the last levels of advancement to come if that makes sense. I'm not sure if that answers your question. I have a decent sized base at most of these locations.

EDIT: My housemate told me I have 8, my computer now tells me I have 12GB of RAM.

I need to know if the lag is FPS drops (ie your game looks like a slideshow or doesn't feel as "smooth") or TPS drops (Ticks per second, it should be running at 20) where you have block lag etc ie you place and block and it doesn't actually place until a few seconds later etc. I haven't been able to reproduce either of these cases

Definitely a drop in FPS. I've now had it crash a few times since then. On one occasion, when it crashed while syncing to a new shell, the new shell was deleted along with its equipment. Luckily it was only wearing red leather armour, but it was still frustrating given red dye was not available near that outpost.