When the built-in app is the right choice, and when it is not
For most households the television's own app is the correct answer: no extra box, no second remote, no HDMI switching. Sets from roughly 2019 onward have enough processing headroom that the app feels immediate. Where it stops being the right answer is on older panels, because a smart TV's processor is chosen for the interface rather than for streaming, and it is the one component you cannot upgrade.
The symptom of an underpowered set is specific and recognisable: video plays perfectly well, but scrolling the program guide is visibly slow and channel changes take several seconds. That combination means the app is fine and the TV is struggling. A cheap streaming stick in a spare HDMI port bypasses the television's processor entirely and is a far better use of money than replacing the panel.
Tizen and webOS differences worth knowing
Samsung's Tizen and LG's webOS both run the app well, but they handle memory differently. webOS tends to keep the app resident, so returning to it is quick. Tizen is more aggressive about closing background apps, which is why a Samsung set sometimes reloads the guide when you switch back from a different input.
On both platforms, turn off the manufacturer's motion smoothing for live sport. Samsung calls it Auto Motion Plus and LG calls it TruMotion, and both interpolate frames to reduce blur. On film and drama the effect is a matter of taste; on fast sport it produces visible artefacts around the ball and the players' edges, and switching it off is the single biggest picture improvement available on a modern set.
Finally, use a wired connection if the set has an Ethernet port and you can reach it. Smart TV Wi-Fi antennas are frequently the weakest in the house, positioned behind a large metal panel, and a cable removes an entire category of intermittent problem.