Texting is very sluggish on 2020 iMac running Big Sur I have a new iMac 2020 64 GB, running Big Sur. The computer has been very good and very responsive. In the past few days, I've run into a problem. Writing a text message is very sluggish. I'm a one-finger typist and I'm pretty slow. When I type a text message maybe the first few characters in a word that appears in my window but I can finish typing a word and look up in time to see the remaining characters appear. When they do appear I'll send the message and have to wait for the send to occur.  I've checked the activity monitor and nothing stressing is happening. I didn't see this behavior with Catalina.  I can live with this but I've never had this problem and I'd like to fix it. Thanks in advance.  The sluggish response doesn't occur while typing this message or emails. As far as I know, this only occurs while texting.
I've tried restarting my computer and it got better for a few hours but started being sluggish again. I don't know what else to try. A site I found said to check the activity monitor for something that was using a lot of memory. I tried that but still no help,
If it's just in Messages.app, I would check two things:

network traffic to/from the app,
try doing an 'inspect' of the process while it is exhibiting the behavior (in Activity Monitor). Do one before it has the problem, so that you have a baseline.
your keyboard connection (bluetooth or USB) and see if something else is interfering with it.

Since it's new, you should also definitely contact Apple, in case there's a hardware issue, which would be a warranty-covered problem. Also, their support specialists are quite good on the phone, in my experience; I've been happy to refer clients there and have not been embarrassed by the support they get. You are entitled to the support, you paid for it, enjoy it.