This doesn't seem like a segment to me. I think an evar would be more appropriate.
When you say you want to see how quickly customers make an order, do you mean how long it takes them to go from the shopping cart to an order confirmation page?
Whenever the user begins to make an order, you could set a data element with a time stamp, and have this data element persist for a visit. Then on the order confirmation page you could have a page load rule that calculates the difference between the current time and the data element, and put this value in an evar.
You'll have discrete values for the time it takes to complete an order. If you have less than 500,000 orders a month this would be fine and you can classify it. If you have more order volume, I would consider bucketing the order speed in the JS of the page load rule on the confirmation page before putting into the evar.
For # of days you could use the method described above. On the order confirmation page you store a data element with a time stamp that persists for the visitor. If there is a previous value in that element you calculate the difference between dates and store it in an evar.
For # of visits you could use the Visit Number. Similar to the above, when someone gets to the order confirmation page you would store the visit number in a data element that persists for the visitor. If there is a previous value you calculate the difference between the visit numbers and store that in an evar.
Can you elaborate on what you consider an "entire" order? A placed order is easy. You just capture the time when the user saw the order confirmation page.
But how do you know when a customer has an entire order in their cart? You could measure the time when they saw the first page of your checkout process. This would give you the time it takes for a customer to go through the checkout process.
You could also measure the time when they saw the shopping cart (either the first time or the last time in a visit). If you capture the last time someone saw the shopping cart, you would get the time it takes for a customer to take an order from the shopping cart through the checkout process, which will probably similar to the time you would get above.
If you capture the first time in a visit someone sees the shopping cart, that would give you how long it takes for someone to go through the shopping process, as opposed to just browsing products. They might go back and edit the items in the cart. This would give you the time it takes for a customer to build a complete order and take it through the checkout process.