Before I went to lipos I ran nimh and nicad packs in my planes. At the time I only had some of the cheaper fast chargers on the market to field charge them between flights. A typical pack would take about an hour to charge. I was/am fairly picky about keeping records on my batteries, recording capacity and other data to monitor pack life. Dropping capacity is a good sign that the pack is not long for this world. I was shocked to find out that several of these packs were dropping as much as 25% in capacity after only about 20 to 30 charges. Cycling the packs at low rates to try to recover some of the lost capacity didn't work. The damage was permanent and directly related to quick charging rates. After I stepped up to better chargers that I could control amp rates on and started treating my new packs with a more gentle charge rate I found they were holding their capacity well past the number of cycles the others had on them.
Fast charging alot will shorten the life of the pack. You might not notice the lost capacity until it degrades even further but it can slowly destroy a pack. More so if you allow it to get hot during the charging process. There's nothing wrong with doing it once in a while, but I wouldn't make it routine. I'll do it when I have to have the pack ready in a few hours.
There are times when fast charging is healthy for a pack. If you find the cells have built up internal resistance caused by crystals they can be destroyed and life restored to the pack by charging at a high amp rate. Some people call this shock charging. They charge at a rate that takes say less than an hour to charge a fully dead pack, or they might even charge at a much higher rate, but only for a few minutes to try to break down the crystal formations. I've heard of this method being used on very old nicad packs that have sat dead for years and won't take any kind of charge. Shocking them back into life so to speak.
Another thing to consider about fast versus slow charging is actual run time. I've read articles about this. A slow charged nimh or nicad pack will often have a longer run time (capacity) from it's charge compared to fast charging it the very next flight. The cells are more fully and evenly charged. In the world of RC where an extra minute or two of power can be critical, it can make all the difference between landing and not making it to the runway.
Most lipo packs have a molex balance plug on them wired in parallel so the charger can monitor each cell's charge seperately. Most nicad/nimh don't and the only way to charge them is in series as they are wired. Over time the cells can get badly out of balance, causing some to be over charged and others to be under charged. More than likely if a cell goes bad you can blame it on this. Fast charging is more prone to causing this problem. That's one of the reasons why it's important to drain a nimh/nicad pack dead once in a while.
The reason why it's critical to balance charge lipo packs is because each cell won't tolerate being charged more than 4.2 volts, or drained below 3V. If they get even slightly out of balance it can destroy the pack or even cause a fire. Nimhs/nicads can put up with some abuse for a while and don't pose as much risk, but it's still bad on them.
There are new generations of nimhs that are sold with fast chargers at places like Walmart. These cells are supposed to be able to take the extra heat and punishment, but if you look up the expected number of cycles they are expected to have over a lifetime it's always lower than a normal battery using a normal charge rate.