WordPress 5.1 very slow on my iPad

Having upgraded to WordPress 5.1 this site is extremely slow. On my laptop loading the editor was taking 40 to 60 seconds. On my iPad I’ve been getting the White Screen of Dashboard. I’ve deactivated many block plugins and it’s just worked for the first time in ages.

So now it’s time to reactivate the block plugins one by one to see if I can find a culprit.