My biggest eyesore of Monero which would definitely prevent mass adoption (and adoption by big stores) is that I can only send a single transaction every 20 minutes. Once a transaction is sent, the sending and receiving wallet need to wait for 10 blocks (roughly 20 minutes) before they can spend the funds. This is due to security reasons. It is especially bothersome for the sending party, as it locks her complete account for that time and prevents her from spending anything.
What if I payed for groceries with Monero but forgot to buy apples? Do I need to wait for ~20 minutes?
Do subaddresses somehow fix this issue? What is the recommended strategy except having multiple wallets to spend from? Will this issue ever be fixed / even is a known issue which is being worked on?
Update: Proposal is on reddit https://www.reddit.com/r/Monero/comments/7qepqi/proposal_add_way_to_automatically_split_utxos/