SE have no control over what would happen when your PC has to calculate and draw 200% more charecter models than now, any lag from brining retainers to the streets would be entirely based on a players PC and connection, unless you want the game offline while they downgrade the graphics to fix this client side lag.
Back on topic, the wards do badly need stabalising, however its not always as simple a job as you all seem to think, it requires more configurations, maybe new hardware for the servers, also it may not be the servers, they could need more beefy infrasructure equipment to the servers that contain the wards. What they need to do is buy a completly new set of equipment they have, build an exact replica of thier setup (isolated from the current network) and copy the configurations over, then they need to stress test each piece of hardware indevidually then they find the bottle neck and can start to find the issue, then they would beable to optimise the neccecery configurations or upgrade hardware.
All of this takes time, take psn, that was offline for 1 month to only upgrade thier security while thier network was offline, all they needed to do was test on the current network, then change configurations and upgrade firewalls and everything else. Of course all of this is speculation but it is what I would do if I was in this situation, also what needs to be taken into account is the amount of staff they put to rebuilding the game because of the bad rep.
Also lets not forget there is still no certainty over the wards because of the amount of people that want an AH, if they spend over 100k on testing and upgrading the hardware for the wards then decide to ditch them it would be a complete waste of money.
So I would say the main reason they havent been stabalised yet is the demand for an AH and the uncertanty over the future of the wards. Until it can be shown that any money spent on the servers that handle the wards can be justified the stability will not change much unless its a simple configuration change which I highly doubt because it would be fixed now.
A way to fix the issue (but it would cost alot of money is to use differant servers for each ward, then when you search the client is programed to send a multicast to all of the ward servers, this would hugely increase capacity and more retainers could be put in them (if the wards get enlarged) the downside is they need to be certain they will be keeping the wards because it would cost over $300k