Since Update to Mojave - MacBook Pro more slow and Safari LAG

In that case here are some troubleshooting steps to follow, in no particular order:

• If Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support
• If Safari doesn't load a page or webpage items are missing - Apple Support
• How to use Internet plug-ins in Safari for Mac - Apple Support
• "Safe Mode": Use safe mode to isolate issues with your Mac - Apple Support
  • Determine if the problems exist in Safe Mode. Then, restart normally.

Needless to say, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" products, anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.

Back up your Mac if you have not done so already. To learn how to use Time Machine please read Back up your files with Time Machine on Mac.

Then, make an appointment to have your Mac serviced because the behavior you describe strongly suggests a hardware problem, specifically a failed startup disk: Contact Support

Please determine if the same problems occur in "Safe Mode": Use safe mode to isolate issues with your Mac.

I have a nearly identical laptop and have the same problems you've got. I'm dying' to know what the problem is. I hope we get an answer sooner than later as it is tremendously annoying.

same, its annoying how freaking slow it is. it's like using a 90's computer again.