iMessage really slow to send/recieve on WIFI

Hi,

iPhone 7 256GB 11.2.1 (have updated many times and issue remained.)

I have been having this problem for a few months now where when I am sending an iMessage to a friend either via wifi or 4G/3G/GPRS sometimes it is really slow to deliver.

There are times where I am messaging someone for over an hour and it is pretty much instant, and then all of a sudden it will be really slow to send the message, or the blue line will complete but the message and woosh doesn't sound for another say 50 seconds or sometimes longer... The person whom I have messaged says that their iMessage is delayed sending through to me also? this can persist for hours or minutes intermittently.

I have turned off my WIFi on my device when this has happened and it seemed to work again for three or four exchanges of messages (through 4G/3G/GPRS) then it seems to be slow and undelivered until I wait a couple of minutes. This can be really frustrating. I find myself toggling my WIFI on and off as well as airplane mode in a hope to refresh things. - Its worth noting this typically only happens when I am at home using my home WIFI or 4G, and occasionally when i am out and do not have a great internet signal. I'm with o2.

Can anyone offer any advice or tips? Let me know if you need any more information.