Clean install of Catalina, then transfer everything from backup "by hand"

Manual? Forget it. Start over, if you’re going to take that path. Haul over your files as needed, rebuild everything else.

The tool used for this is Migration Assistant, which knows where the pieces are located, what the new attributes and protections should be, and which can prompt for which subsets should be copied.

How to move your content to a new Mac - Apple Support

Usual choices for slowness can be hard disks and particularly failing hard disks, insufficient free storage, add-on anti-malware, add-on cleaners, add-on VPN clients, SSDs operating without trim, among other choices for performance degradation.

Thomas Yap wrote:

My MacBook Air system took up more than 200 gb of storage space, what can I do to free up the storage space?

Please start your own thread, as this is an unrelated question, and the resulting mixed threads tend to get me and some other readers confused.

When you post that, please include some more detail on your Mac model and year, its storage configuration, what capacity displays you’re looking at, what you’ve tried and what then happened...

200 GB is either a bloated macOS install or a tiny install, depending on what you’re looking at.

A half-terabyte is locally considered a small hardware configuration, too. Macs with less than a half terabyte are best left to those with no digital archives, few photos and music and related media, and mail-deleting habits far more ruthless than mine. The lowest-tier configurations are probably best for schools and businesses, as most of us seemingly accumulate more stuff.

My MacBook Air system took up more than 200 gb of storage space, what can I do to free up the storage space?