On Monday I bought a item on sale from a online website and they had a buy 1 / get 1 free deal for the day. I placed my order and I paid with bitcoin, but they have a 10 minute timeframe for the transaction to confirm.
So, I did the checkout and I paid with Bitcoin and watched the countdown from 10 minutes. Well, the SegWit transaction did not confirm in 10 minutes.
In the end, my money was refunded and I missed out on the deal.

The Lightning Network would have solved this problem, but we are not even close to mainstream adoption and the merchant says that they not going to switch to the Lightning Network, until there are large demand for it.

Yes, the main thing made me concern in bitcoin is the transaction speed.
There are many peoples already using bitcoin nowadays, but they need to wait their transaction accordingly with the btc blocktime.
Lightning network is the solution to overcome the slow block time in btc. If it fully developed, everyone can use bitcoin without waiting for 10 minutes.