We use AWS to host our site, and shortly after I increased memory from the standard 30gb to 40gb on a t3.medium, I noticed that the POS we use (magestore) was acting super slow. Where it would normally take just a second or two to ring something in, and move to payment, was taking closer to a minute. Usually, when the POS is acting slow, it's because there are a lot of user sessions taxing the server due to a bot or ddos attack. But this is different. No matter how much traffic the site is experiencing, the same consistent lag persists on the POS. Once I submit an order, the receipt would normally pop instantaneously, but now it religiously takes 5 seconds to produce a receipt - I can count it down in front of the customer with 99% accuracy.
Some time later I found out that the shopping cart function is experiencing a similar lag. It takes 30-45 seconds to go through each step.
With both POS and Shopping Cart, it doesn't take long to add something to the cart, it's the process of completing the order. The POS doesn't have an integrated payment portal (we use PayPal Zettle on a tablet separate from the POS), so it's nothing with the merchant services.
Otherwise, the site is super fast. Clicking through the frontend or backend, the pages load almost instantly. Thinking it might have been a resource issue, I upgrade to a t3.large instance on AWS but no change other than an even faster backend and frontend.
We're in the process of migrating from M1 to M2, so I have been simply dealing with the slow POS/Shopping Cart. However, the project is taking longer than expected and I'm getting tired having to joke with the customer while we wait on a receipt.
What could be making the cart function slow, but the rest of the site fast?