What's the oldest gen iPad you would run Shopify POS on?

[SRSM]

We recently switched to Shopify POS and we're learning that our gen 6 iPads are not quite sufficient to operate the app without major issues. We're experiencing crashing as well as extremely laggy loading in searching products and during checkout.

We also have an iPad 10 and an iPad Air 5, both of which run the app smoothly with few complaints, but it's a tall order to have to buy 3 brand new iPads like that. We've found a good price on older open-box and refurbished models, but I'm worried we'll run into the same issues. 

Would anyone be willing to share the oldest generation of iPad you're running Shopify POS on, and whether you experience any performance issues?