If 400 causes issues, 200 works better and 100 was smooth, then I would say its what I already described above.
Add to that your 8GB of memory and macOS Tahoe using more memory, coupled with Safari within Tahoe having performance issues, coupled with the 3rd party library the Bric uses not using an optimized performant approach. It can be the perfect storm (no pun intended) for some users and devices to experience issues.
I would be curious regarding the mention of “stuttering and glitchy” on iPhone 15 Plus and iPhone 15 by @AdieJAM and whether those phones are running iOS 26 which presently has the same / similar Safari issues as macOS Tahoe.
Back to the snow – for instance 400 flakes - means 400 divs, 400 instances of filters and opacity, 400 @keyframes animations, plus 400 instances of .snowflake:nth-child & @keyframes totaling approx: 5200 lines of CSS for 400 snowflakes. Then in the JS upon resize it redoes it all again. 
Hopefully someone understands the ^ implications concerning performance shown here. Without the need for testing on all their clients devices to see how it may perform, don’t assume.
So without getting further into the actual performance technicalities of the above method used by the pure-snow.js library that this Bric uses, I will instead again say – it’s far better to use Canvas for such effects. 