A Samsung that takes four seconds to open Netflix feels like a TV on its way out. It usually isn't. Tizen caches aggressively and never really cleans up after itself, so a set that was quick two years ago is now dragging a couple of gigabytes of junk around behind it. Clearing that takes about two minutes and doesn't touch your accounts or your apps.
Start there before you reset anything.
Clear the cache
On any 2020 or newer set: Settings > Support > Device Care > Start Device Care. Let it finish. It clears cached files across every app and it does not log you out of anything.
You can also go app by app: Manage Storage, pick one, choose View Details and then Clear Cache. That path only exists on 2020 and later sets — on anything older, resetting Smart Hub is the equivalent.
If you can't find either, check both places. On newer firmware Self Diagnosis sits inside Device Care rather than beside it, and Samsung has moved it more than once.
If the option is greyed out
This one catches everyone. Device Care and Self Diagnosis grey out while an app is still running in the background. Nothing is broken. Back out to the home screen, close whatever's open, and the option becomes available again.
Then pull the plug
Not standby — actually unplug it, and leave it for 30 seconds. Standby keeps the memory alive, which is the whole point of standby and also the reason it doesn't fix anything. A real power drain clears the background processes that Device Care doesn't touch.
Between clearing the cache and a proper drain, most slow Samsungs are fixed. If yours is, stop here.
Work out which kind of slow it is
If it's still sluggish, the next step depends on a distinction worth making, because the two have nothing to do with each other.
Apps buffer, menus are fine. That's the network. Check All Settings > Connection > Network > Network Status (older sets put it under General instead) — and get onto 5GHz if you're on 2.4, and if the router is on the other side of the house, move it, extend it or run a cable. Nothing on the TV will fix a weak signal.
Menus themselves stutter. That's the TV. Move on to Smart Hub.
Reset Smart Hub
Settings > Support > Self Diagnosis > Reset Smart Hub. It'll ask for a PIN — if you've never set one, it's 0000.
This clears out the app platform and its accumulated state. You'll sign back into your streaming apps afterwards, which is mildly annoying and takes about ten minutes, so it's worth doing properly rather than repeatedly. It's a bigger hammer than Device Care and it fixes things Device Care can't.
Full factory reset is below that again, at Settings > General > Reset. I'd only go there if Smart Hub reset made no difference at all, because it costs you every setting on the TV and rarely achieves more.
The part nobody wants to hear
Some of these TVs are just past it.
Samsung ships modest processors and small storage, then app developers keep making their apps heavier. A 2017 set running a 2026 build of YouTube is doing something it was never specified for, and no amount of cache clearing changes the hardware.
You can tell you're at that ceiling when everything is fast for a day or two after a Device Care run, then degrades again. That's not a fault you can chase. That's the TV filling up faster than it can cope with.
The fix, and I'd genuinely do this before buying a new television: put a $40 streaming stick on a spare HDMI input and stop using the built-in apps. You get a current, fast app platform and the panel keeps doing the thing panels do, which is display pictures. The TV's own software becomes irrelevant. It's the best value repair in home cinema and it takes five minutes.
If your set is also refusing to install new apps, that's the same underlying storage problem, and clearing the cache is the same first move.
FAQ
What's the quickest fix? Settings > Support > Device Care > Start Device Care. About two minutes, and it doesn't sign you out of anything.
Why is Device Care greyed out? An app is open in the background. Close it and the option comes back.
What's the Smart Hub reset PIN? 0000 unless you changed it.
Apps are slow but the menus are fine. That's your Wi-Fi, not the TV. Get it onto 5GHz or wire it.
It's fast for two days then slow again. You're at the hardware limit. A streaming stick on a spare HDMI port is cheaper and faster than replacing the set.