The plugin slows down my site a lot. Sometimes the cart page just freezes.

You need to go to Pricing rules > Settings > Calculation and enable the “Disable shipping calculation” parameter.

Mini cart opens on page load

Our plugin have to rebuild cart contents to get correct prices. But some themes/plugin try to show mini-cart when new item was added to the  cart. You should use this PHP code to fix the problem:

Total price not correct due to the quantity of product cant be equally divided

You need to enable the option “Woocommerce” -> “Tax” -> “Tax options” -> “Round tax at subtotal level, instead of rounding per line”.

Update prices when customer clicks the quantity boxes in the cart

You should use separate plugin to add this functionality.
I suggest to try

Free product can’t be added to the cart. I see message “Sorry, this product cannot be purchased.”

WooCommerce verifies product before adding to the cart. So this product must be published, in stock and has price defined.

I can’t change quantity or delete item from cart

It’s a conflict with another plugin which modifies cart items too. You should turn on debug bar and send us report/json file. Read short guide here.

I marked checkbox “Add products to cart at normal cost and add coupon…”, but I don’t see any coupons in the cart

You should visit “WooCommerce” -> “Settings” and mark “Enable the use of coupon codes”.

How to allow customer to select free product

You should create package rule and set zero price for free product. Please, check 3rd example.