Screen Share (via Messages) is slow as molasses on Catalina

I have been experiencing “slow as molasses” screen sharing with iMacs upgraded to Catalina (latest version).
The screen sharing is activated via the running on both iMacs.
The extremely slow refresh makes the screen sharing not viable to use if one iMac is running Catalina.
This has happened with several friends whose iMacs are on fast internet connections. (My iMac is still running Mojave 10.14.6).
I had this confirmed this week. One Tuesday the screen sharing speed was as expected – both iMacs were running Mojave. On Tues night my friend upgraded to Catalina. Mine stayed on Mojave.
On Wednesday, started a screen share session again … and the screen refresh rate was totally unusable.
Interestingly, two of my other friends using MacBook Pro or MacBook Air computers (with Catalina); the connection works fine.
(Apart from, most times, after scrolling, it can take a long time for any responses to get up to zero lag time).
I’ve been using ScreenSharing via this method for years and its only with Catalina iMacs that this super slow response time has become evident.
Anyone have a clue as to what is happening?

Seeing that no one has responded, it seems my experience with slow screen response is isolated to my environment, so I’ll investigate further. It may be the type of router in use (supplied by Telstra, Smart modem, Technicolor or the Arcadyan model) has a hand in this.
These same routers don’t work nicely (sometimes) with older Xerox printers using Ethernet.
Happy for anyone to suggest any options. Or to confirm screen share has worked perfectly with Catalina.

My experience with Networking in Catalina has been truly awful - from missing or non mounting local servers, to slow or failing copy operations to screen shares dropping randomly (and slowness sometimes) …I hate to simply bitch and moan…but wtf Apple? I am sticking (mainly for email/Audio-Graphic-Text production) to High Sierra until OSes become stable. And only supporting Catalina for new machines with my customers.

Does Catalina still support AFP? In my Mojave machine, I can still connect to AFP drives (instead of smb shares), though I’ve read Apple doesn’t officially support the AFP protocol.

(p.s., Screensharing doesn’t use either protocol, I don’t think, but I have a mixed OS network, and the High Sierra machines Screensharing is so much more reliable)

Screen Sharing uses the VNC protocol.

I haven’t used it since upgrading to Catalina, so I can’t comment on the speed. In general, I’d expect the network connection to be the bottleneck.

I see in my Catalina Sharing pref that I can still choose to serve using AFP. I myself have however not yet actually tried it on Catalina. But the pref being there indicates to me it is intended to still work. /knocks on wood

Catalina supports AFP. I use it all of the time to connect to work. Got to replace those work servers with something newer.

I use my Catalina to remote control other Macs running macOS 10.14 or earlier with no problems. I have not controller another Mac running Catalina.

@ace That only works when on the same network, from what I read?
My OP relates to when the Macs are in different locations, using their own modems (such as private homes).

When connecting via Messages app (which initiates the connection), it is actually then handed off to the app Screen Share which resides in the “Hidden” folder of System Library, CoreServices, Applications Folder (which holds a variety of other apps).
I keep that in the Dock so I can see Recent Items to start screen share sessions with previous clients, without using Messages to imitate the connection.

I did a short test using TeamViewer with a client which was showing a slow as molasses connection (where previous to Catalina, there was no lag), and got the same slow speed.
So that points back to modem problem. (Both modems were showing 80-100 Mbps download speed, so it’s not speed, but something else is a problem).
Bring on Big Sur asap, Apple and banish Catalina!

In Terminal, run nettop to see the processes with network connections and the source and destination ip addresses and port numbers (nettop needs a wide Terminal window, use up-down arrow keys to scroll process list, press q to quit). My guess is when you’re using Screen invoked by Messages that the process is connecting to the VNC port (5900) on an Apple Messages server that acts as a data relay between your Mac and the remote Mac that’s serving up a view of its screen.

The connection to the Apple server may not be on port 5900 since the VNC protocol can work on any valid port number, you’ll be able to tell which one it is in nettop by the amount of data coming in (especially if the remote Mac plays video or does something else to make the screen contents change a lot).


I’m pretty sure that the Screen Sharing app (which, as you note, is what handles screen sharing regardless of how you initiate the connection) uses VNC regardless of whether it’s an in-LAN session or one that goes out over the Internet. VNC doesn’t care about the transport medium.

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