Disabling SMT in the AMD Ryzen 9 3900X does have some benefits in improving average frame rates in a few titles bringing the processor much closer to the results posted by the Intel Core i9-9900K at stock settings, according to game benchmarks run by TechSpot. However, tests show that turning off SMT only offers gains within the 5% expected deviation and the 1% low scores, in fact, seem to favor enabling SMT.
https://www.notebookcheck.net/AMD-Ryzen-9-3900X-gaming-benchmarks-reveal-turning-off-SMT-narrows-the-performance-gap-with-the-Core-i9-9900K-only-in-select-titles.428628.0.html