Slow site loading

I have been having problems with the speed at which my wordpress site loads https://sherwood-archers.org.uk/ anything from 10 to 60 seconds. I have used pingdom https://tools.pingdom.com/fpt/#!/cjHp2R/https://www.sherwood-archers.org.uk
The main problem seems to be with admin-ajax.php I have looked at various posting an topics but so far have not found anything that helps me resolve the problem. I am not using any kind of commerce plugins and I am using a plugin called Ajax heartbeat tool and total cache but nothing I have tried so far seems to work.
Has anybody got a solution for this issue and if they have can they please exlain in the very simplest of terms how I can fix this.