Having a character sheet open causes slowdowns and stutters across my entire computer

If I have any D&DBeyond character sheet page open, my entire computer slows to a crawl after 1~2 minutes.
Closing the tab with the D&DBeyond character sheet restores performance to expected levels.

As a measurement, I am running two 165Hz screens and playing on Foundry.
Plays perfectly smooth with no DDB charsheet open.
With a DDB charsheet open for over ~2mins, plays with massive stutters - panning cameras, virtual dice etc feel like 10fps at best.
Opening / Minimizing windows and dragging them around, gifs playing in discord etc all feel like 10fps.
Close the DDB charsheet tab and I'm back to butter smooth 165fps everywhere within 5sec.

Whether the DDB charsheet tab is open or not, CPU and GPU usage hover around 10-20% and nothing there looks indicative of these slowdowns.

Hardware - 7800X3D, RTX 3060, 64GB DDR5-6000
Software - Win11, Firefox 149.0.2

Would appreciate help in diagnosing what exactly is going on there, as I enjoy playing and dming with charsheets open on the side, and that is currently practically unplayable for me.

My advice:Test it w/o Foundry.

Because that isn't a 1st-party product.

Do you have any other tools that integrate with the DDB character, like Beyond20 or other browser extensions?  

Me and my friend (4 of the 5 people) all using firefox have the same problem. Extreme lag only on the character sheet. If you close the tab with the character, everything comeback to normal.  When you re open it, it take about 5min and everything lag again.
So we are kinda closing and re opening our character every 5-10min.

Keep in mind that we never had any problem before the update of the website recently.