MQTT Delay

Hi Guys,

I am just beginning to get into arduino.
I have done a fair bit of research myself before posting on here. I am on the mind that normally someone else has had the same problem and searching uncovers the answer so I very rarely post on forums. I just sit in the background watching and reading.
I have a problem I cannot seem to find the answer to though.

We have just sold our house and are looking to build a new one and have arduino running a smart house, lights, locks that sort of thing.
I am trying to get my arduinos to talk to the MQTT server on my home server.
Everything seems to be working ok except that there is a fair delay on the arduino when I have more than one connected to the server at a time.
If I have 1 connected the response is almost instant, once I have 2 or more there can be up to a 60s delay from the publish to the arduino reacting.
I have tried to narrow it down to the possible culprits. I have swapped computers to see if it was the server. Different routers, though both are netgear but different models. Different LAN cables. Different amounts of arduinos, currently have 3 for testing and it's anytime I add more than one.
I have set a pin on the arduino to "turn the light on" to see if it is a slowdown on the arduino side but that works instantly everytime, even with an incoming message.

I don't know what I can do to fix it.
Has anyone else had the same issue? Is this a quirk of arduino with MQTT?
Should I try a different protocol and if so what one?

Thanks in advance, any help is appreciated as I am now on a tight schedule since our house has now sold.

You have posted no code, and no facts. All you have is some handwaving that says that multiple clients cause a slowdown.

We don't know if the problem is your server takes too long to react to a client, or exactly what the problem is.

So, the odds of your getting help are pretty slim. There are ways to enhance those odds, without resorting to little blue pills and spam forever.