It has been proven that it can run on 12 years old AMD CPU on 256MB RAM.

http://www.techradar.com/news/software/ope...e-proof-1288287

So whatever heaviness you're experiencing might not be the Windows, but your own system.

1. Check the task manager what process is hogging what resources. It could be either memory, disk or CPU. Most of the time it is the indexing services reading your disks hogging the CPU and HDD bandwidth causing the slowness.
2. If you do upgrade, you might want to check all drivers and update them to Windows 10 version of the driver. Some driver does cause issues like Killer NIC causing memory leak without the Windows 10 driver.
3. If you do upgrade, you might want to do clean installation. Historically, Windows always have some sort of error when people do upgrade. Despite Windows 10 is the cleanest I've ever experience, a clean install might help your issue.

I would imagine in your situation, it comes down to few things

1. Early driver version issue. Not to mention AMD/ATI nowadays is really famous with their slow driver update.
2. Early build issue. I've experienced this with my GTX680 on Win8 preview previously. It certainly could be coming from early driver bugs as well but never take early preview build as an indicator for final build because the whole system (OS and drivers themselves) are not mature enough.
3. Your system itself. SDRAM is pretty old... A lot older compared to first gen DDR (while DDR is technically an SDRAM, I'm assuming it is SDR SDRAM and not DDR). I would imagine the CPU your system running on is a lot less capable compared to the one that the author of the article use.

But again, we also need to admit that the PC in the article doesn't run with good performance as well at 256MB. While it booted up fine, it took 41sec to open file explorer.

I would really love to try Win10 on an old PC just for the sake of reviewing the experience, but I don't have a working old PC...