There's no two ways about it... no exaggeration about 8 out or 10 summoning chamber spawns is a lag lockup, We've done it every way...
Hold off til last second and enter with a full raid party getting the most empty instance is the only consistent way to succeed, but in that case you need a good raid party that can solo levers and even solo Altars because you will get one guy who drops or didn't have a key or otherwise leaves you short. (also make sure you are all in the level range, or some of you will end up in a higher or lower level instance)
How to fail:
Mass invis on Altars confirmed 1 Arcane in each altar in party voice: still fail
Spam AOE's still fail
Not spam AOE's still fail
No pets no summons still fail
Only one person allowed to AOE altar still fail
It's server side resource usage related lag for sure. Purely a numbers game.
Ways to succeed:
1) Spam mass healing/cures on altar with everyone pressing attack and "playing through the lag" as though everything you're doing is working (even though you aren't seeing results) occasionally like 20% succeeds.
2) Get lucky and you don't get any leg in the instance you joined now all you need to do is "not suck"
3) Holding off with a good raid party and going in late so you get an nearly empty instance.
4) play at 4am EST when Turbines servers are least loaded.
On technique 1) this is exactly like old Shroud lag, where even though you aren't seeing the green numbers you still spam masses, because you know the server is getting them, it's just not reporting it back to you. If you lag you can self heal still as long as your client shows you a casting animation. Only cast a normal amount (like 1 firewall at your feet and then hit your Necro blast, and refresh your Aura, or one BB and then mass cure critical, followed by mass cure serious) make sure they are quickened so you don't get an interupt you can't see!. You can't see your AOE but it's there and it's killing stuff and drawing aggro off your Altar, so just "trust in the shroud" and heal through dropping the occasional AOE... the times this doesn't work I suspect are more because most people throw their hands up and give up when they can't move or see red/green numbers... Just pretend they happened and don't stop spamming cures until you see "the Dragon is slain" or "your key has been returned" Basically pretend you're are blind, or a veteran of shroud lag days
Mabar: lag spam like that guy you know