Mac Pro 5.1 Mojave "Lagging"

Apple hardware test was installed on the dive that originally shipped in your Mac, and was also available on the original release DVD, if it had one.

If your drive was ever erased or replaced, Apple Hardware Test has likely been lost.

A user has collected as many as could be found easily, and posted them on Github with a procedure for a making bootable USB thumb drive:

https://github.com/upekkha/AppleHardwareTest

Macs after about 2013 use Apple diagnostics, loadable from the Internet.

Download and run this non-invasive "discovery" utility, Etrecheck. Launch and before proceeding, set its preferences to "allow Full Disk Access". Allow to run and use the third Icon from right in the forum "Reply" window (ToolTip "additional text") to post the entire report. No personally identifiable information is included in the report, not even the actual names of your Volumes:

Using EtreCheck - Apple Community

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I did not see any well-know bad actors.

You should connect you backup drive and let Time Machine run at least every-other day. All it wants to do is an incremental backup, but if you wait more than a couple of days, it can't take good advantage of a system-maintained list of changed folders. When that list goes stale (typically after a few days) it has to spend about four hours re-computing what needs to be backed up.

Time machine is intended to run automatically at low priority in the background, without slowing down your important work.

You CPU usage is wonky:

Top Processes Snapshot by CPU:

Process (count) CPU (Source - Location)

com.apple.WebKit.WebContent (6) 103.07 % (Apple)

Other processes 27.38 % (?)

EtreCheck 3.50 % (App Store)

com.apple.WebKit.Networking (3) 0.67 % (Apple)

Safari 0.54 % (Apple)

I dunno what pages you have open, but you need to close them when you want responsiveness, and open them again later if you need them. Sometimes its not the actual contents but the Advertising that drives those numbers up so high.

I have a comparable machine running with 8 pages open, using only 16 percent CPU, but I make a point of closing any sports sites or news sites, because they use up too much CPU. Then I look at them again later.

Most of the hangs over the last 7 days are related to CPU usage.

Hello,

Lets try running Apple Hardware Test. Also please verify that in System Preferences -> Users -> Tap your user account -> Login Items that there are no unnecessary login items. When your Macintosh starts up it will open these items when you login and can slow down your system.

Apple Hardware Test

Hello,

Do you have a mechanical spinning HDD installed or a SSD?

Lagging and responsiveness are two terms for the same thing.

Your Mac is so busy servicing web pages and the ads they contain, your screen lags.