Does any one know, a transaction with a large fee applied, will it definitely be confirmed very quickly?even with the blockchain congestion at the moment? or is the congestion having quite an impact on transactions regardless? 
If the fee you use (calculated in sats/byte) places your transaction in the top 1MB of transactions in the mempool... chances are good a miner will include your transaction in a block... You can see the fee spread vs. total size of transactions here: https://btc.com/stats/unconfirmed-tx
Bear in mind, that others are coming along adding transactions after you while you wait for a block to be mined... so don't aim too close to the 1MB mark, as if you do, and 10,000 people come in and put a higher fee than yours, you might end up dropping down the list...
In the screenshot above, if you were to put in a fee of around 160-170 bytes, you'd be around the middle of the top 1MB of unconfirmed transactions... That should help increase your chances of getting in the next block mined. Of course, nothing is guaranteed... but it will help.
Also, you can see how if you use a fee of around 100-120 sat/byte, you going to end up in the 2MB to 8MB range... and could be waiting for quite a few blocks before your transaction gets confirmed.
EDIT: And this screenshot illustrates my point about not aiming too close to the 1MB mark... by the time I finished writing up my post, the distribution now looks like this:
