In Google Analytics under Audience > Behavior > Frequency & Recency you can find the count of visits report. Add the Built-In segment Visits with Transactions. This way you can see how many visits it takes for people to purchase. Compare year over year count of visits for purchasers. There is a good chance the amount of visits it takes for people to convert has gone up year over year.

A few thoughts on this:

• As the internet becomes more ingrained in our daily routines we all browse more. A visit to an ecommerce site is no longer a signal of high purchase intent like it used to be.
• Retargeting is more important. Visitors are taking more time comparing prices and sites. While they’re weighing their options it’s worth it to remind them of your offering with display ads.
• Do you offer discounts so often that visitors waiting for your products to go on sale?