You may have heard somewhere that poor cable management inside your PC's case can block fans and air pathways, making your CPU and GPU's temperatures rise. This video puts that theory to the test, and proves that most mid-size cases do just fine with or without decent cable management. In this test video from the LinusTechTips YouTube channel, host Luke Lafreniere shows just how little of an effect your internal cable management has on the system's overall cooling performance when you're using a decent mid-size computer case. After setting a base benchmark, Lafreniere added a few cables for one test, then a whole mess of cables for another test, but there was no rise in CPU or GPU temp. Even after placing a stuffed animal, a t-shirt, a Santa hat, a roll of duct tape and a clothing iron inside the PC case in addition to the cables (please do not try this), there was still no change in temperature.
The bottom line: good cable management looks cool and keeps your machine more organised and easier to work on, but it's probably not going to affect performance in any way. Having a good case and the proper cooling components are what's really important regardless of where you put your cables. Of course, as Lafreniere explains, the setup he's using in the video is already well-designed for cooling, so take these tests with a grain of salt.