i have a ryzen 5 2600 cpu and i have been finding that my cpu struggles with simple tasks or old games that i played. on task manager it says i have only one core although i have six. i updated my motherboard bios before taking action btw.
have you checked in task manager that the CPU graph is set to "logical processors" by default I believe its set to "overall processors" . you can change this by right clicking on the CPU graph and choosing "logical processors" could you have put the CPU cooler on too tight? I have known people who have had RAM issues due to installing a CPU cooler too tight.
Your OS runs in a custom safe-mode, automatically enabled to disable multi-core-processing, due to an incompatibility of the the drivers handling pci-e and dma, in most cases as a reaction to a not correctly initialized (i/a)GPU driver. Disabled multi-core in such processor-class leads to one active native core represented by two logical units, which are partitioned ressources - handled similar to basic hyper-threading.