Wifi speed dropping

briancl wrote:

IBooting into safe mode has been suggested, but Mac will not do that. (Restart with shift key just reverts to 'Normal').

Grateful for any ideas...

If you have a rechargeable wireless keyboard connect it to your Mac with the charging cable and then try to boot into Safe Mode again holding down the Shift key until the  logo appears on the screen.

Don't be 100% sure it is Apple.

If it works in Safe Mode, but not in Normal Mode, it is typically caused by some software you've installed. To help troubleshoot, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Okay. Then I'd suggest shooting us the EtreCheck report that Jack suggested earlier.

I noticed you restarted you modem did you also do this in the following order:

1. Unplug modem from power source and restart and wait until fully booted.
2. Unplug router from power source and restart and wait until fully booted.
3. Restart Mac in Safe Mode then restart normally and re-test

By chance is there any third party antivirus, cleaning or ANY other third party maintenance apps installed?

briancl wrote:

Sorry guys - wifi issue cured when runnung in safe mode but still there when I revert to 'Normal'

That tells us there's some 3rd party software that might be the culprit. So download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.