https://developers.google.com/speed/pagespeed/insights/
On 27th May 2020 I had used page speed where I got a pretty good score for desktop (90+) and for mobile around (85+), but as on 28th May 2020 the metrics seems to drastically changed, I can see the PageSpeed has new version (v6) but no proper release notes are provided here https://developers.google.com/speed/docs/insights/release_notes.
Anyone had faced similar issue and found that google pagespeed did undergo certain upgrades then please provide me some references if possible.
After some digging I managed to find the draft of the proposed scoring weights.
https://web.dev/performance-scoring/?utm_source=lighthouse&utm_medium=wpt
The large shift in scores is down to how the weightings have changed
As you can see there is a massive shift to emphasis on Total Blocking Time (TBT) (JavaScript execution time mainly) and when the Largest Contentful Paint (LCP) occurs (presumably as this indicates large shifts in the page layout / visible content that may be distracting / is a good indicator of when the above the fold content is fully loaded (as opposed to showing a 'spinner')).
They have also added a third new metric Cumulative Layout Shift (CLS), a metric that works out how much the page layout 'moves around'. This has a low weighting at the moment but I imagine it is part of a larger plan to ensure all late-loading assets are captured that affect the 'above the fold' content and may cause frustration (trying to click a link to find an advert loaded in and moved it for example).
These huge changes in weightings and introduction of new metrics are the cause of massive score decreases you may experience.
I can confirm that my site that used to score 99 or 100 now only scores 87 so it is indeed a large shift in how they are scoring. They also now seem to take into account SVG rendering as my site is 100% SVG driven yet scores low on the LCP, this is something they did not initially take into account with the LCP stats.
For now focus on the two articles I linked on TBT and LCP as those are the new metrics they are choosing to emphasise making up 50% of your score.
As OP pointed out in the comments the main changes for the new PSI v6 are located here