It is, recently, when a new update is downloading, I cannot watch Vids on Steam at the same time and even the Store is lagging.

The only difference I'm aware of is they changed the compression algorithm from LZMA to zstd for new content / updates (things uploaded to Steam by developers before the change remain compressed with LZMA).

The reason for this change is that zstd uses less CPU time to decompress data compared to LZMA. Or, to put it another way, with a fixed amount of CPU time, zstd will decompress more data.

One possible outcome of this is that if your downloads used to be CPU-bound, they may well not be any more, and something else might now be the bottleneck, like your disk or your internet connection.