I don' t know how many people already had important bitcoin transactions stuck into the UNCONFIRMED state for days or weeks. This should happen if you put a low fee into your transaction, but basically it can happen anyways. The fees are taken by miners for the mining work. Mining is the computational power required for the system to survive. The system gives an award to convince people to use powerful machines for mining. Mining has been made difficult on purpose with the PoW (Proof of Work) to avoid that everybody could become rich in a moment with mining (and the currency useless). Basically, to mine transactions your computer must generate hash codes. The system requires a particular kind of hash code, for instance on Bitcoin the system requires a certain number of zeroes inside the code. If the condition is not respected, the code is rejected and you can add a redundant header into the payload, that is called "nonce" and that is used to generate a different hash code from the same content. The code is generated until the system accept it and the miner can get the fee only if the block is mined. The condition that must be satisfied determines the level of difficulty, that is tipically measured with the number of attempts required for an hash code before being accepted. In this moment, the difficulty level of Bitcoin is 1,590,896,927,258. This is the reason why transactions require expensive machines like Antminer S9 or a mining rig of eight GeForce 1080 Ti. Just to make $24 per day you need to generate 14 TH/s with a consumption of 1350 W, just because who created this system had a misconcept of freedom and power. Even if you may find the idea of mining fantastic to make easy money, the mining itself is one of the worst concept ever created in the history of computer science, a total waste of energy and the reason why your transactions may get stuck into an UNCONFIRMED state for days or weeks, because the system is not efficient at all.
The system is wasting continents of horse power just to produce basically nothing and to move money in the worst way possible. To perform few transactions per second, the system may require billions of hashes that wouldn't be necessary if Proof of Work never existed. But you may say: without mining, people don't have any reason to run their computers and make the system alive. This is wrong on so many levels. First of all, the level of difficulty today is so high that mining is not profitable at all for common people but for big companies. So what was born to be decentralized, now is centralized again in the hands of powerful companies, that currently detain the biggest slice of computation power required by the system to work. And second, the idea or rewarding people to run computers and make the system work is wrong. If you remember how internet was born, in the past we had computers maintained for free by people in some university. Then the routers spread all over the world creating the internet of today, made of companies that guarantee the service. The same is happeing to Bitcoin, with the difference that internet doesn't require any kind of PoW to work.
With Bitcoin the power now is on the hands of companies that are making millions while people are slowly losing their power because they don't have the hardware for mining. Probably in the future Banks could buy all the companies that in this moment are mining Bitcoins, to centralize the currency again or make it disappear. It was pretty obvious that it would end this way. If Proof of Work requires power, it's natural that power is provided by companies that are the most powerful groups of people in the world. Fortunately there are other altcoins that are fixing the problems related to the concept of mining and PoW. Don't get me wrong, I still believe that cryptocurrency is the best, but even if everything started with Bitcoin and Bitcoin in this moment is the most powerful crypto in the world, it won't be the currency of the future because it will be replaced by cryptocurrency that don't have the same problems and that implements a better concept of long term decentralization that can be maintained without wasting power.
The advantage of a cryptocurrency without proof of work:
1) Mining can be done for free by volunteers. The hash are accepted at first try without proof of work. The system is optimized for working and save power consumption.
2) If compared to Bitcoin, even a laptop can make the same work of a mining farm.
3) Banks could buy mining companies, but it would be useless. Even without companies, people can run computers to mine efficiently without wasting power, while with Bitcoin this is not possible because PoW and the outstading level of difficulty that makes mining more suitable for companies than for people (good job, you wanted to decentralize money and created the final intrument for the future centralization and a waste of power that can damage the planet).
Probably this a cryptocurrency without PoW is the currency of the future. There are already new currencies that are implementing a similar concept and they are very promising. I believe that even if Bitcoin is the best now, it will die in the future and all the capitals will be moved into more efficient and decentralized currencies.