Windows Antimalware Service Executable slowing down Firefox

I'm having the same issue over the last 2 weeks, but my issue affects all operations, not just browsers.

I’m running Windows 10 Home on an SSD, fully updated. Version 1803, build 1734.191 on an Intel 6600K with 8 GB RAM. Windows and all apps on the SSD. Data confined to a standard HDD. A plain vanilla system that rarely uses even 3 GB of RAM.

Symptom: Slow response to many attempted operations. I can't even scroll a Word file or a web page. I first noticed this issue 2 or 3 weeks ago.

I notice this regularly throughout the day, perhaps 10 or 15 times, seemingly at random. This PC is often idling and is rarely under a significant load, so I’d guess it occurs many times without being noticed.

The browser (usually Pale Moon, occasionally Chrome) can be closed or open.

Task Manager shows that CPU usage by Antimalware Service Executable can stay in the 28 to 32% range for 20 seconds or so, by far the heaviest load on the CPU.

I assume a scan or indexing of some sort is going on. Disabling real-time protection in Windows Defender seems to help a lot, but real-time protection apparently turns itself back on in a day or two.

I also use Malwarebytes, but its services don’t seem to ever use more than 1 or 2% of CPU and I’ve never noticed any conflicts between it and Windows Defender.

Even though 70% of CPU is supposedly still available, the PC is largely unresponsive to any mouse input until and unless the Antimalware Service Executable CPU usage drops back down to its normal range of 0 or 1%.

I might see this occur when idling with browser closed and nothing but Task Manager and one Word file open.

Or it can occur when I attempt something specific, such as opening an application or web site.

For instance: 6 or 8 seconds to open Excel (normal time would be under 2 seconds); 6 or 8 seconds to open CCleaner or Task Manager (normal time would be virtually immediately).

Googling tells me I’m not alone, but I don’t see a huge avalanche of recent postings about this problem.

Nor do I see an authoritative solution from trusted sources.

This is the most annoying issue I’ve had since installing Windows 10.

Any ideas or suggestions?

Some points here:
High CPU usage from Windows Defender - Windows 10 Forums
How to Fix High Windows Defender Disk Activity When Using Mozilla Firefox

Seems there's a problem with 1803.

Yes, if you turn off real-time scanning from the GUI, by design it is again magically enabled.

You have to work harder to do that, or install 3rd party AV.

Turn On or Off Windows Defender Real-time Protection in Windows 10 | Windows 10 Tutorials
Options 6,7

One of the complaints about Defender in 1803 more widely (can't recall if it's mentioned in those links I quoted) is that of high CPU use when it scans its own folders or files.