After a random amount of time after reboot (20 minutes to 2 hours), my laptop would have considerable lag, locking up for 2-3 seconds every 6-10 seconds. I tried to follow What is the cause of a high CPU usage of 'system and compressed memory' in Windows 10? because the symptoms were similar, but the root cause was different.
Observing Task Manager during these lockups, 'System and compressed memory' would be the only thing spiking and would reach about 25% before returning to a negligible level. Following what magicandre1981 did in the other thread, I used WPRUI to capture the activity and opened it in WPA. I drilled down the CPU spikes through ntoskrnl.exe -> ndis.sys -> rt640x64.sys. Looking up that driver, it was for the onboard NIC (Realtek).
Updating to the latest drivers from Dell didn't help. Since it's a laptop and I'm not likely to use the wired NIC anytime, I just disabled it in Device Manager, and the problem stopped immediately.
I thought I'd type this up since I spent so much time researching the problem. Big thanks to @magicandre1981 for his work in the other question and for leading me towards WPRUI and WPA.