Magento 2.3.5 site that loads pretty quickly generally, checkout maybe a few seconds to load but that seems par for the course for Magento 2
However, the issue is that after pressing 'Place Order' the loading icon literally shows for 2-3 minutes before loading the success page.
This happens if using OneStepCheckout, the default one page checkout or regular checkout and using Braintree or Cash On Delivery plugin. The only shipping method is 'Free'
What I have noticed is that we get an order confirmation email fairly quickly (15-20 seconds) and then an invoice always arrives exactly 2 minutes after the order confirmation - and it is after this that the success page loads.
We don't need the invoice to be sent automatically, and I guess this is contributing to the excessive load time - is there any way of stopping this?
Any other ideas for what could be causing this ridiculous load time? I've checked debug.log / system.log while it's happening and literally nothing shows