Lagging When Opening Website

I have updated my ThinkPad Helix (1. gen) to Windows 10 as soon as possible, went well, no hassle except to find some drivers, but got them in a matter of days. Anyway, I didn't experience the improvement of performance, I expected, so I went for a clean install. Also because I couldn't get Office365 activated - a glitch, I read about several places, none with other solution than clean install of Windows. And I prefer a clean, virgin, fast and clutterfree installation - until I clutter it again ...

Installation went well, took a little over two hours (with cleaning of SDD), chose to keep no files (but had to start over again, because I wasn't satisfied with the folder structure and user names). But now, up and running again, I'm experiencing lags and holds, when primarily, opening at website with Flash or other things going on, like video or a chat window. The cursor reacts with seconds delay, hardly to control, have to use my fingers to close the browser again. And it's the same, regardless using Firefox, Chrome (this is worst), Edge or IE ...

Closing the window or tab with the problematic page, the computer responds quickly as i would expect it to...

Tried to disable the native flash-player in Chrome, didn't work. Went from Chrome Canary back to regular Chrome, didn't work.

Also I had CPU or GPU (onboard) suspected, but installed all relevant Intel management software, got Lenovo System Update and got system specific software and drivers, but left out the bloatware.

I can vaguely remember same problems, when I went from W 8 to 8.1, but can't remember the problem and the solution. I think, I did a reset, but not sure. Another clean install? Took me hours, so would like to avoid this...