Yes, you are basically correct about losing more speed with leaning than with edging. Why is this so? I.e., what is causing the boat to slow? The answer is the resistance of the water to the object moving through it.
Your kayak will generate the least resistance when gliding straight forward. Any turning motion will increase resistance by itself, and the sharper the turn, the greater the resistance. Additionally, the altered shape of the submerged part of the heeled kayak (compared to its submerged shape when level) glides less efficiently, & will further increase the resistance of the water to its movement. In general, the more you heel the boat, the more the resistance will increase.
You can heel a kayak more by leaning it than by edging. Most good paddlers that I’ve seen can hold a sea kayak edge of about 30 degrees or so. Someone who is very flexible, with excellent balance & proprioception can do better than that, but I’ve never seen anyone hold an edge of 45 degrees or more–around that point, and certainly past it, it becomes impossible to keep your body mass centered over the kayak, and you will capsize. In contrast, with a decent brace, you can heel your boat over to 90 degrees or even past 90 degrees–very briefly, until you stop moving–at which point you must pop up, or convert to sculling, or capsize.
If you heel the boat more, you’ll bleed speed faster–so leaning–associated with more heeling than edging–will slow you faster. In addition, because you’ve added your paddle to the mix–by bracing–you’ve added water resistance to the paddle blade, even if it’s just “skimming” the surface. So, leaning has a couple of factors that will generally increase the water resistance and slow you faster.