Plenty of people notice their Mac feels different with a second screen attached. Fans spin up sooner, apps take a beat longer to switch, and the overall feel is less snappy than working on the built-in display alone. That's a real, explainable effect, not a placebo.
Unified memory means your display is drawing from the same pool
Apple Silicon Macs use what Apple calls unified memory: a single pool of RAM shared between the CPU, GPU, and every other part of the chip, rather than separate dedicated memory for graphics like older Intel Macs with discrete GPUs sometimes had. This design is genuinely excellent for a lot of tasks, since the CPU and GPU can share data without copying it back and forth. But it also means that anything the GPU needs to do, including driving the pixels on every display you have connected, competes directly with your apps for the exact same memory your web browser, Slack, and everything else are already using.
Every additional display adds its own frame buffer, the block of memory holding the actual pixel data being sent to that screen, and drives additional GPU compositing work as your Mac renders windows, menus, and animations across two separate surfaces instead of one. A higher-resolution external monitor, particularly a 4K or 5K display, requires a meaningfully larger frame buffer than your Mac's built-in screen, and running one alongside your laptop's own display means your Mac is maintaining two of these simultaneously.
Why it's worse than just "using two screens"
It's not simply that you now have more screen space to fill with windows, which would already use somewhat more memory on its own. Extending your desktop across two displays means macOS is compositing and animating a noticeably larger combined canvas continuously, even when most of that canvas is sitting idle showing a desktop background or a mostly static window. That ongoing compositing work is small per frame, but it happens dozens of times every second for as long as the display stays connected, and it draws from the same finite unified memory pool as everything else you're running.
What makes it worse
A higher-resolution or higher refresh-rate external display raises the memory and processing cost further, since more pixels updating more times per second means a larger frame buffer refreshed more often. Running two external monitors instead of one roughly doubles this additional load again. And if you're also running memory-heavy apps like a browser with dozens of tabs, a video editor, or a virtual machine at the same time as a demanding external display setup, the combination is what typically pushes a Mac from "feels fine" into visibly sluggish territory, since the display overhead and your apps' overhead are both drawing from the same pool at once.
What actually helps
Match your resolution and scaling to what you need. Running a 4K or 5K external display at a lower scaled resolution, rather than its absolute maximum, reduces the frame buffer and compositing overhead somewhat, though the difference is generally modest compared to the effect of unified memory sharing itself.
Reduce how many displays are active when you don't need them all. If you regularly work with the lid closed and an external monitor, or occasionally with two external displays, disconnecting the ones you're not actively using during memory-sensitive tasks (like video export or a large data process) frees up that overhead for your actual work.
Keep an eye on what else is running alongside your monitor setup. Since the display overhead is fixed for as long as it's connected, the more control you have is over everything else competing for the same unified memory pool: excess browser tabs, apps still open in the background, and anything you're not actively using right now.
Why this catches base-model Mac owners more often
A Mac with a smaller total memory configuration, say the entry-level 8GB or 16GB options still common on older MacBook Air and Mac mini models, has proportionally less headroom left over once the display overhead of an external monitor is accounted for. The same external monitor that a 32GB or 64GB Mac barely notices can meaningfully eat into the available memory on a base-model machine, which is part of why this specific slowdown tends to show up as a surprise for people who upgraded from a laptop-only workflow to a laptop-plus-monitor desk setup without also reconsidering how much memory they actually need for that new pattern of use.
It's not a fault in your monitor or your Mac
It's worth saying plainly that none of this reflects anything wrong with your specific external display or a defect in your particular Mac. It's simply the direct, predictable consequence of how unified memory architecture works when you add a second, often higher-resolution surface for the GPU to continuously render and composite. Recognizing that helps set the right expectation: the fix isn't a repair, it's understanding the trade-off and managing what you can control around it, which is everything competing for memory alongside the display itself.
Where a memory-relief tool fits in
Shiny can't reduce what your external display itself needs, that overhead is a fixed cost of the unified memory architecture for as long as the screen is connected. What it can do is free up everything else stacked on top of it, closing orphaned background processes and reclaiming memory tied up by apps you've already quit, so your Mac has more genuine headroom to work with alongside your monitor setup rather than running right at its limit.
For more on how unified memory works across the whole system, see Unified memory explained.