The problem with your testing regime is that the mysterious slowdown may only happen every several weeks and then not always for the same device.
While it can happen at any time of the day of night, I most often get this late-ish, around 11:30 pm - midnight. I looked at the backup schedule and saw it was set to 2:15 a.m. So, that shouldn't be cause, unless how it calculates the time zone is off somehow but per this screenshot, everything looks right:
The only thing I'll mention about the settings is that I just put in my zip code the other day, but the time zone has been set to US/Eastern forever (and I live in the Orlando, FL area).
In any event, I changed it to 3:30 a.m. and every other day.
I now have a similar problem as @Matt658 but i'm on a C7. Matt did you resolve this with the help of bobby-D? its extremely frustrating. Also, i'm not sure if this is a coincidence or not but i cannot get my Alexa integration to connect. It's been rock solid for 4 years but the last few weeks it has been intermittently disconnecting, similarily my severe hub loads have been appearing across the same timeframe. It may be a coincidence but is there a link here @bobbyD.
This is driving me crazy, i want to upgrade to a C8-pro but i'm reluctant until i sort this and get some stability back. If i have the same issue on the C8pro, then WAF is going to disappear in the blink of an eye 
@stueyhughes, I ended up have two separate but possibly related issues and both have been resolved. Apologies to all for not marking this issue as such, it was something that resolved itself over time and getting back to this thread to update the results fell off my radar.
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Issue #1 - After looking at the engineering logs, Hubitat support confirmed that my original C8 was in fact facing an actual hardware malfunction and they replaced the unit since it was still under warranty.
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Issue #2 - After receiving the replacement unit, I was still having some severe load issues, but not to the the degree of the original defective unit. After doing a lot of digging, it seemed that the issue was stemming from something on my network, but I couldn't pin it down (more below).
Initially, I had my C8 hardwired directly to a UniFi Security Gateway (Model: USG). It was connected to the LAN2 port on the USG. At first, I thought it was jumbo frames causing the issue, as that has been reported multiple times as causing issues with the C8. I ensured that jumbo frames were turned off and I also set the link speed of the port to 100/100. Even with these settings I continued to have sporadic issues.
Fast forward to today, during the course of network upgrades, I have since swapped my USG with a custom built pfSense box and instead of hardwiring the C8 I decided to switch over to WiFi. Since switching to this new setup, I no longer have any issues whatsoever with severe hub load, and the C8 has been running strong. While I cannot say for sure since I never went back to validate, but based on the turn of events, my guess is that my issue was stemming from the C8 not liking being hardwired directly to my USG for some reason. If you're not already, I'd try connecting your C7 via WiFi instead of hardwiring to see if that helps.
Hopefully this helps, and I'm happy to provide any additional information that might be helpful.
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Just as an aside. Jumbo frames cause the network to lock up on the c5/c7 and C8....I would put a sniffer on your pc and mirror a port to what the hub is connected to and see whats coming in and out. A network connection shouldn't be overloading the hub as that is usually an app or driver unless it's one of your lan based integrations that's causing it.
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