With i9-9900K you should be able to upsample to EC7/DSD256 with no issue, and I have seen many do it with i9700K. Have you try setting "Multicore" to "Auto"? Another thing to check is using the Intel's Turbo Boost. In the BIOS, make sure to set turbo to "on". With i9-9900K, I see users running EC7/DSD256x48 at 4.7GHz, so your 4.8GHz is more than enough. Meanwhile, have you try different filter to make sure the stuttering isn't coming from the heavier filters?
In my case, I have the i9900KS, no graphic card, used to be 8GB and now 16GB 2666 RAM, with everything set to Auto in BIOS, running AL/HQPe. CPU temp is around 60C, so no issue with thermal throttling.