Mail app very slow to open and respond to mouse clicks due to fontd - Sonoma vs El Capitan

The problem happened anew last night. Mail in El Capitan has ground to a halt, with the fontd process taking up 99.8% of CPU for 20+ seconds after each mouse click.

I can see an email has arrived, with white space where preview text would normally be shown. It's from an insurance company. I'll attach a screen shot.

I worked through the very slow responsiveness of Mail, believing it was induced by this email and used View, Raw Source from the drop down menu, to find this in the email's code.

Here we see a large block of code for non-binding spaces and white joiners. This is the code referred to as the Preview Text Hack, discussed in the thread that is linked in the original post and promoted by Litmus. I'll copy those links into this post.

mac email Inbox slow to open - Apple Community

https://litmus.com/blog/the-little-known-preview-text-hack-you-may-want-to-use-in-every-email

The slowing down of Mail is solved by moving the guilty email into Trash. I'll keep it for the moment so I can show it to the insurance company's CEO and others who might be interested.

I'll be curious to see if this company changes their approach to using this Preview Text Hack, once informed of the problems it causes for their clients and prospective customers.

Hopefully this thread will help others who find that Mail in OS X 10.11.6 El Capitan and possibly other operating systems becomes very slow.

Hello PRP.

Thanks for your intel on Sonoma not having exhibited the slow-down effect I've described. That's good to know.

I can't comment on the language used in the Litmus article, but I can assure you that the problem of fontd hogging 99.8% of CPU is real, at least with Mail in El Capitan. I'm waiting to see it surface again when I receive another email from the company that caused the problem. When that happens I will ask for your help with some forensic examination of the email's headers and coding. I expect we will see strings of zero-width non-joiners (‌͏ and ‌) and non-breaking spaces ( ). Please stand by. I don't know how long it will be. :-)

This discussion will attract the attention of anyone else who experiences the problem and comes looking for clues. When they do they should use Activity Monitor to see if the fontd process is using 99%+ of CPU for many seconds, coinciding with Mail in El Capitan stopping in its tracks with each mouse/trackpad click. Please report your findings.

Thank you Leroy and PRP for looking at this.

in my original post I included a link to an earlier Apple Discussion thread. In that is a link to the Litmus webpage where the Preview Text Hack technique is discussed and promoted. I’ll paste that link here.

https://litmus.com/blog/the-little-known-preview-text-hack-you-may-want-to-use-in-every-email

So there I was thinking I would see replies along the lines of, ‘Yeah, we all know about that old fontd problem from preview text hacking. It was patched over with update X.X.X … ’. But apparently not. Perhaps I need to ask a broader question:

Is Mail in recent MacOS releases prone to drastic slow-downs, possibly due to the formatting of emails received?

Thanks.