XPS 15 9570 CPU throttling, extremely laggy

I recently bought an XPS 15 9570 (4k, 16GB). The laptop performed reasonably when I first inspected it, but when I took it home it became clear that it is almost unusably slow. Around 50% of the time, the laptop will run reasonably well with CPU and GPU usage not reaching above 10% during light use. The rest of the time, the fans start to whirr and the CPU begins to throttle at 0.79GHz, while the built-in Intel GPU usage shoots up to 90-100% in Task Manager. When that happens, the whole system chokes up: basic tasks like using file explorer and opening the start menu become extremely laggy and I get a very low frame rate. Even typing or opening a new browser tag is noticably laggy to the point that the laptop is not usable.

I read that removing the battery for a few second before replacing it can help reset a throttling issue which causes this behaviour, but that didn't work. Also, the RAM usage doesn't drop below around 50% even when idle with no apps open- is that normal? I also read that applying thermal paste migght help resolve the issue.

The laptop is running the most recent version of BIOS (1.13.0) and Windows 10. Running User Benchmark shows that the NVIDIA GPU seems to be behaving fine, but the CPU and RAM are massively underperforming, particularly the CPU. I heard the throttling at 0.79GHz and related behaviour might be caused by a known issue with some drivers, or BIOS?

User Benchmark scores:

UserBenchmarks: Game 26%, Desk 49%, Work 33%
CPU: Intel Core i7-8750H - 36.7%
GPU: Nvidia GTX 1050-Ti (Mobile Max-Q) - 29.5%
SSD: Pc300 NVMe SK hynix 512GB - 138.2%
RAM: Samsung M471A1K43BB1-CTD 2x8GB - 66.9%
MBD: Dell XPS 15 9570

Any and all help would be greatly appreciated.