It can hugely affect shifting but the biggest thing is simply aging and dirt not the housing quality. If the cable is simply inexpensive it would be of no consequence but if it is old and frayed internally or dirty or worn inside the cable housing it can absolutely be an issue. Better than a very expensive set of cables changed infrequently is simple, inexpensive cabling that is changed somewhat regularly (each season?). Worn cabling housing can show itself by becoming more difficult to change gears, particularly up front where it requires more pressure to go from a small chainring to a large one and an even bigger clue can be a failure to fall down to a smaller, harder cog in the rear. The solution does not need to be expensive cabling. The difference between new, basic cabling and very expensive high end cabling is pretty darned small for most people. Proper routing with proper lengths of good, decent cables should be all that is required even if your cables are old. I would not bother popping for super expensive stuff if money is a concern but if your cables are more than two years old you might well need new cable housing. Any good lined housing will be fine. If you have -any- visible kinks...areas that show they are not a nice easy curve...then you know for sure it's time to change them. You can try a drop or two of light oil on the cables...there are videos that will show you how to release the tension on the cables so you can move the housing around and oil the cable. Sometimes you can reuse the cables but most shops will recommend you just replace the cables with the housing. Don't buy the most expensive stuff around or the cheapest junk. Good housing with a slippery inside will be just fine for anyone but a serious racer. A good example of decent housing would be Jagwire Road Pro.... nothing special but it does have a teflon lining and will be just great. This is also something you can do yourself with minimal tools.