Power Query is drastically slow (sometimes, in some files)

Windows 10, the most recent version of Power BI (but I had the problem earlier), 16GB RAM, Core i5 CPU, SSD. Everthing works great and really fast than suddenly it becomes drastically slow (in the same file with the same data and queries). The simplest query takes many minutes. I mean really the simplest, like removing duplicates from a table with one column and 2 identical records. It loads CPU at 100% for many minutes.

Backgrounds downloads disabled. Cleared cache. Reboted computer. Nothing helps. Copied the query into a new pbix file and it work as expected - I see the result of preview generating immediately, while it's still strugling in the main file. And it can be like this during many attempts and then just suddenly it works fast again (same file, same data, I didn't even closed the file, it was very slow, then suddenly it works fast). Had this problem with other reports in the past. Power Query works normally then with no visble reason it stucks on basic operations. No other heavy applications work in the background. Something is wrong with Power Query itself.

What it's doing? Some caching? It looks like a bug, probably I need to contact Microsoft, but maybe someone experienced this problem.