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The limescale buildup explanation covers most of it, but there's also a good chance your heating element itself is just degrading over time 😅 Even without visible deposits, the element's resistance can increase as the metal ages, meaning it needs more electrical energy to produce the same heat. If descaling doesn't help much, you might be looking at replacement time - they're usually pretty cheap and easy to swap out though!
Have you checked the heating element? Over time, mineral deposits and limescale build up inside the kettle, especially if you have hard water, and this layer acts like insulation between the heating element and the water, so it takes longer to transfer heat!
Guys hit the main culprits but there's a sneaky one they glossed over - the thermostat or cutoff switch can wear out and start triggering too early, so your kettle shuts off before the water's actually at a rolling boil, just warm enough to flip the switch. Combined with limescale on the element slowing heat transfer, you get a double whammy. If descaling doesn't help much, the internal switch is probably getting tired and it might be worth replacing if you're handy, otherwise the kettle's done for.
Mineral buildup is the most common culprit, but I'd actually check if your kettle's power cord or plug connection is getting loose - mine was taking forever to boil last year and I realized the outlet connection was shot, so it was drawing way less power than it should. If that's not it, yeah, limescale definitely insulates the element, and sometimes the element itself just wears out electrically even without visible deposits. Worth descaling with vinegar first since that's the easiest fix, but if that doesn't help you might just need a new kettle.
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