My laptop is increasingly taking a long start-up time. How can I fix this?

When switching on, it takes longer than 15 minutes until the device is ready.

Replace hard drive with SSD. They are now available from 20 euro. You just have to reinstall Windows and save the data on another data carrier beforehand (but you should do it regularly anyway).

2 things that really bring something:

Swap HDD to SSD
Reduce autostart programs

But even then, a start time of over 5 minutes is extremely rare.

Maybe run a clever antivirus over it

I just think the laptop will be 10-15 years old?

HDD? - Defragment

SSD / HDD - Remove contaminated sites with Windows "clean up", then defragment the HDD

Autostart - many programs / apps that start with Windows start reduce what starts everything with you can see under Windows 10 in the task manager

Pets - run a proper, thorough anti-virus scan on the entire system

SSD / HDD damaged clusters - Either with Windows own tools (CheckDisc), or with tools like CrystalDiskInfo or HDTune read out the smart values (Crystal Disk Info), and / or check the SSD / HDD for faulty clusters (HD Tune)

CPU throttling - device is overheating, the CPU goes down to the bottom with the clock rates for protection. You can actually hear the fans turning for their lives. Then clean up the PC.

So back up data regularly, don't constantly reinstall Windows.