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.