About once a month Blue Iris will get this bug.
Typical CPU usage for Blue Iris will be around 1-3%. When this bug is happening, the CPU usage for Blue Iris will be 23-25%.
While this bug is happening, the computer comes to a crawl. Logging in takes about a minute; opening any application is nearly impossible. Blue Iris becomes very slow and almost unusable.

When this bug is happening, Blue Iris will start getting "live.sem stuck" errors, "alert image not created" errors. Today it did both of those, but now also it's getting "Disk can't keep up" errors.

No single solution will fix the issue. It's happened probably 5+ times in the past year.
Sometimes multiple reboots will fix it.
Sometimes rebooting all cameras will fix it.
Sometimes a Database Delete and Regenerate will fix it.
Sometimes updating Blue Iris to the latest version will fix it.
If I disable cameras, the issue goes away little by little.
I can do all those things, and sometimes the bug will persist (like right now).

If I stop Blue Iris, the computer works normally again.

System is up to date on drivers. SSD's are all healthy according to SMART. The computer is ONLY used for Blue Iris, nothing else is running.

Task Manager is your friend.

Can you confirm if you have anti virus running, and if you have set up exceptions for BI5 ? See Help file if required.