Since I saw a lot of questions about slow transactions with the Ethereum network, I would like to summarize the issue and try to help new users...
The Ethereum network can be congested depending on the number of pending transactions. Here you can have an idea:
https://etherscan.io/chart/pendingtxIn order to have your transaction sent fast, you need to set a "reasonable" gas price in gwei. The higher the gas price, the more you will pay for the transactions but you will ensure to have your transaction confirmed in a reasonable time... In fact, miners will confirm the transactions with the higher gas price first...
When the network is not congested 1 or 2 gwei is usually enough to have your transaction confirmed in minutes. If the network is busy, you can maybe wait for hours/days...
So the best way I found is to check:
https://ethgasstation.info/It suggests you a reasonable gas price in gwei in order to have your transactions confirmed in minutes based on the network status...
For example, now the suggested price is 10 gwei.
If your transaction is stuck (pending for hours/days) you can then try to cancel it and relaunch it with a higher gas price. You need basically to send to yourself a transaction with 0 ETH and the same nonce. Here is a guide with myetherwallet:
https://kb.myetherwallet.com/transactions/check-status-of-ethereum-transaction.html 
