Interesting; I found the opposite. Network speed (at least, in practice, like manipulating files in a large folder with thousands of subfolders and files) has improved a bit over the 6.11 series, esp. 6.11.5. In pure file serving terms, those were the worst ever for me, and 6.12.2 (never tried 6.12.1) is noticeably better. The physical speed (using OpenSpeedTest docker webpage interface from windows and mac clients) is about the same (Aquantia AQC107 10Gb NIC), but actual practical file usage/serving has gotten back to where it's only semi-painful.
Nothing like 6.83, of course, which remains the high-water mark for me in file-serving response and speed. Every once in a while, I roll back to 6.83 and am amazed again at how much faster and more responsive it was with exactly the same hardware (true on 2 different machines). All the cool new capabilities now are great, but its a shame that, in general, the performance of the original and most important function of Unraid, network file serving, seems to get worse and worse. I realize some of that is due to underlying Linux changes; hopefully this slight uptick is the beginning of more improvement.