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I think some All-in-one PCs are not much bigger than a LCD monitor. But, smaller All-in-one PC may use mini-pc as the computer instead of using laptop and desktop PC parts to build the All-in-one PC.

In size they can be a bit as they are very heavy and should you need to move it often you risk breaking your whole system.

Some of those All-in-one PC have faster CPU, RAM and video chips, so I feel they need a bigger heavier metal heatsink cooling fans to properly cool the computer.

This is true those all-in-one systems can be heavy for some people who have weak muscles. Using a cart with wheels and straps to securely strap the PC to the cart can help move the all-in-one computer.

they can be pretty fast, though without a good ssd their speed don't hold up for long.

Using a lightweight Operating System like Linux Mint and Debian Linux may make an All-in-one conputer with a hard drive feel faster.

Hybrid hard drives with a small ssd built-into the hard drive are faster than regular hard drives. There are also hard drives which spin at 10000 RPM which feel faster.

It would help, but most of these are deployed in work environments that don't use linux.

Dual booting Windows 10 and Linux can be useful. A lot of programs like LibreOffice, Firefox, Chrome, etc now work on Linux. Users can boot into Windows when they need to use Windows-only software like MS Office.

Maybe using a lightweight version of Windows 10 like Windows 10 S would make All-In-one PC feel faster.