What's the hardware difference in different speed writers?

What are the real differences between say 24x, 32x, 40x, 48x, 52x etc, etc CD-RW recorders?

I am asking about the real hardware difference. Not the drive firmware.

It depends:

Taking the ever popular Liteon’s, there were several chipsets which demonstrate it well - the Mediatek chipsets - I don’t know the exact names, so I’ll use the Liteon-style designations:

3S chipset - used at 24/10, 32/10 - and later a single 40/??
5S chipset - used at 32/12, 40/12, 48/12 - though other makers did 16x RW with it, though this is overspeed on HS media, NOT Ultraspeed
6S chipset - Used at 48/12, 48/24, 52/24 - the first US-RW capable one
7S chipset - Used at 52/32

There is a maximum speed that any particular chipset can do, and early models don’t stretch it to the maximum - for CD-RW media, there is a second limitation of media type.

So the firmaware, and possibly the pickup unit (higher speed needs more laser power) and the motor, determine how fast withing the chipset capability the drive will go, but the chipset sets a hard limit to what is possible.

In the main, it’s marketing, they don’t want to launch the top of the range first, unless it lanches at a high price premium with a more affordable model lower down.