Very slow booting and lag

There's a lot of reasons this could be the case. More than likely it's your hard drive. If it's a mechanical drive, then your PC speed will be heavily affected by the slow hard drive. You may also need more memory.

Several options,

Overheating (dirty or malfunctioning fans or exhaust ports), disassemble and clean, repaste the cooler with a good thermal paste, kryosheet or thermal phasing tape.

The laptop/pc has a conventional old school Hard Drive and it is either on its way out or in very bad shape, replacing the HDD for an SSD Solid State Drive will benefit greatly the speed of booting and overall use, you can simply clone the drives and keep all your files as they are, but working way faster on an SSD.

The PC/laptop has a lot of programs that start when windows starts, they sneak in 1 by 1 and with time they end up slowing your boot time by a lot, simple disable those programs option to start when windows starts. Teams, Skype, zoom, most game launchers, music streaming programs, etc, you should see which ones are on after pc boots up on the bottom right of the screen where the time is displayed.

Windows is just tired (corrupted) you can run registry cleaners (CCleaner free version does it) and clean the registry from unnecessary files, run a disk check for errors on the windows files, or simply re-install windows... a fresh installation is always a good way to start.

I hope this helps.