That was an old saying but they’ve since improved that by a whole lot.
Thing is cPanel was originallyed designed for dedicated servers which had much more processing power and meant for managing tons of clients. Nowadays, most people running a control panel are doing it on a small virtualized VPS server. There was a short overlap in the industry when cPanel was still not yet optimized to be run on smaller dedicated servers (VPS’s) but they’ve since remedied that.
Nonetheless, cPanel is indeed heavier than some other control panels but still quite lightweight when you consider everything it does. To say cPanel is heavier than other panels is like saying a car is heavier than a bike. It simply does many more things that other panels do not. There’s also the issue that many shared hosting servers using cPanel are doing it on stock Apache configurations which aren’t very high-performance whereas other lean VPS-oriented control panels are using lightweight NGINX which are faster out of the box. So basically, it’s not a fair comparison.
But if all you’re worried about is whether cPanel will slow your site down compared to other control panels, the answer is, “No, it will not”.