Yes, that is generally true. But it compounds quickly if there are multiple tracks or transitions, and your editing laptop is a potato.
Sometimes, the input format may be out of your control. For example, if someone used a mirrorless camera to capture video, and the camera insisted on making LongGOP H.265 video, then they’re stuck with that format. They could transcode it with Shotcut’s “Convert to Edit-Friendly” feature to change the format to DNxHR or whatever, but it’s probably not worth the effort. If editing is done on proxies, then conversion is worth even less because the editing phase would rarely use the converted files. Editing would typically use the proxy files (except maybe a full-resolution spot check for any color grading and text alignment).
But sometimes, you will get a chance to take control of the input format. For instance, if someone has old family video that is in an interlaced format and they want to convert it to progressive format for editing, then an opportunity exists to do that conversion directly into a fast format like DNxHR. I’m not saying that’s always the best idea because DNxHR makes huge files that will blow up your archive space. But the option does exist.
It’s mostly about the algorithms used by each format to compress video, followed by how well the codec is implemented for a specific type of processor (x86 vs x64 vs ARM for example). DNxHR, by an intentional design choice, decodes fast by using a relatively simple compression algorithm (simple and few math steps), but that simplicity means not much compression is applied, which is why the files get huge. Meanwhile, H.265 uses a wide array of complex math tricks to achieve a lot of compression. The resulting files are small, but it takes a ton of CPU to undo the math and get the original frame back. These are intentional choices made by the designers of each format. This is why each format specializes in a different stage of the editing workflow. DNxHR is good for editing where speed counts. H.265 is good for final delivery to a customer where small file sizes count (which lowers network bandwidth costs for streaming providers).
Comparing video codecs is one of those fields where somebody can get a Ph.D. in it and still be wrong or surprised half the time. Once they figure it out, the encoding software gets an update which scrambles the results all over again.
Shotcut has a lot of default codec choices. They are good defaults for most purposes. If you have a specialized purpose in mind, you could search the forum for past discussions or ask a new question. Google can bring back a lot of information, too. Experimentation is the real test, but you would need a good background in FFmpeg to set up those tests. So for now, I’d say don’t worry about it until you absolutely need something different.