OK, did the upgrade to Windows 10 today.
Took roughly two hours to download and install.
First experience was terrible: start menu not opening, task manager not responding.
I tried this solution for the first problem without success.
Thru my Rainmeter process monitor I noticed that MsMpEng.exe kicked in quiet often with high CPU usage. This opened my eyes that apparently Microsoft Security Essentials is part of Windows 10 !? Since I had Avast Antivirus installed on my system this was a good cause for trouble: two virus scanner competing with each other for disk resources, a great way to slow down the system to almost a halt.
The turnaround then was to de-install Avast Antivirus. After another reboot I finally was able to open the Start menu and use task manager and overall system behaves much smoother now.
The Windows 10 installer should consider this and ensure not two virus scanner will be running in parallel. It should either prompt with information that Microsoft Security Essentials will be available with Windows 10 and offer to de-install any other installed virus scanner, or offer to disable Microsoft Security Essentials if user prefers to stay with his current virus scanner.