GoKawiilThink of a smart TV a little bit like you would a PC. A cheap PC is good for light browsing, but you wouldn't expect to play modern games on it because the hardware isn't powerful enough. Like a cheap PC, a low-end smart TV might have the same apps as an expensive model, but a faster processor can make all the difference.
That processor is responsible for a lot of the heavy lifting involved in your TV can properly playing what you want to watch. Before displaying anything, the TV has to decode the video it receives. The dedicated system-on-a-chip that powers the TV's smart functionality does this, turning (often compressed) video streams into the frames you're able to see.
Once it's done that, it can start working on the picture. It might need to upscale to make lower-resolution video fill the bigger, 4K-capable panel. A cheaper smart TV will often lack some of the advanced processing found on higher-tier models, such as AI-assisted upscaling that tries to reconstruct fine detail as the image is enlarged.
It's all down to the system-on-a-chip that powers it. A more capable processor is able to enlarge the image while keeping it sharp, but cheaper processors with weaker upscaling might leave it looking soft or with harsh edges. The end quality will differ in everything you view on the display. The screen's refresh rate is another factor, which is most relevant for gaming. Low-end TVs are limited to 60Hz, while better TVs can match the 120Hz output modern consoles like the PS5 are capable of (in compatible games).
There's also a more obvious sign that you're using a cheaper smart TV: menu speed. A less powerful TV will be slow to respond, especially in apps. It can even cause some lag as the processor is pushed to its capable limits. While expensive smart TVs aren't immune to this, they are typically more powerful, so you shouldn't see slowdown — at least not for a while.