Not just slammers. Other units suffer from this too. See bug FS#3312. Also more info in: Units stuck at the teleporter Teleporter gets jammed This will probably get fixed at some point.
Units that consume the space of others to move tend to clog it. I think it is because teleporters tend to force units to walk to one point to use it, and when they are done walk to another point to "stop". If they jam AFTER using it, they are just as bad, because they will fight to reach a "leave teleporter" point in front of the teleporter and they all shove too hard for any single one to reach it and become able to do anything else. My experience stems from making leviathans "teleportable". If it weren't so tactical in "NavalRedux", I would make leviathans unable to. Bluebottles cause some problems but only in real mass. Anything not so wide have no problem, most units really, even small naval boat units.
