WiFi slowdown (or cap) on Catalina 10.15.2 - but not with SafeBoot and iPhone

As a precaution, before the EtreCheck test I removed the third party extensions Little Snitch and PreyProject (an anti-theft utility that already helped me get back my stolen Mac in 2018). However, I want to say to peter_watt that the anathema "third party extensions are no good" is not always true. I have been using LS and Prey for MANY YEARS, and they worked fine on Catalina 10.15 and 10.15.1 without ANY problems, so IMHO they won't suddenly be creating a problem.

As suggested this is the EtreCheck report, plus a screenshot of the Network section.

In addition to this, I have played around with ActivityMonitor and discovered an interesting fact: force-quitting "mDNSReponder" once after a reboot… resolves the problem completely. I did this around 2 hours ago, and a SpeedTest now gives me absolutely perfect results (screenshot nr. 2):

I have re-installed all my habitual software (LittleSnitch, PreyProject and others) and can now confirm:

• after a reboot, speeds are low as previously described
• force-quitting "mDNSReponder" once after reboot (inside ActivityMonitor) resolves the problem completely., i.e. gives me back normal WiFi speeds with rates that touch almost 100% of what my LTE can deliver.

After a quick search here and on Google I discovered that "mDNSReponder" has a long history of failures and successes. This story here seems to confirm it is still one of Apple's biggest problem zones.

Hello SohailAfghan and thanks for the endorsement!

Yesterday (Jan 27, 2020) I set up two brand-new MacBook Pros - a 15inch i9 late 2019 and a 16inch i9 late 2019. Both machines came out of the box. Both machines have no problem when receiving internet connection via USB-C Ethernet adapter. But both machines show the exact same problem on WiFi: capped speeds that do not get higher than 15-18Mpbs (no "capping" with upload speeds), and both react "favorably" - meaning giving normally high speeds on WiFi - when I force-quit mDNSResponder.

Today (Jan 28, 2020) I brought both machines from the Austrian countryside (and the LTE network) to a superfast cable network in Berlin, Germany. The result is exactly the same: download speeds on WiFi are capped to 15-18Mbps on a network that achieves 250Mbps in download and 50Mbps in upload.

Both machines run on Catalina 10.15.2, and were setup from scratch (the 15'' with a clean install, the 16'' from its box).

A quick update: after installing the Catalina 10.15.3 Combo Updater, the problem completely disappeared. I must therefore deduce that it had been entirely created by the previous version of macOS, 10.15.2.

Thanks for still getting back to me with this! When I said "third party" I meant apps that plant extensions in the Library and System folders - but I get your point. However, it seems at least odd to me that now, with everything re-installed and force-quitting mDNSResponder just once after reboot, the Wifi works perfectly. Would logic not dictate (!) that, as you say, "TeamViewer, Transmit, Microsoft AutoUpdate, etc. no doubt use mDNSResponder" and therefore slow down the Wifi?

Agreed. But I did a clean system install only days ago because of this annoying problem. And the only third-party software I did re-install on the new Catalina was LittleSnitch…