Extreme lag in composer when replying to HTML mails

Since I started using Mailmate in 2017 I noticed a bit of lag when writing in the composer (sometimes text appearing 1-2 seconds after writing), annoying but no disaster. With some recent update (can't exactly remember when - within a month or two), it went completely berserk: When I write, the text slowly appears on the screen - slowly as in 1 character every few seconds appearing on the screen and lines scroll like I am on a 300 baud serial line (plenty of hardware available). So I have to resort to an external editor and copy text back (I could probably configure an external editor, but I prefer the built-in one).

The problem only appears if I reply to an HTML message. It may be due to malformed HTML - on a quick test it is a problem on all Outlook and Apple Mail messages as well as Mailchimp newsletters I could find while not a problem with any Mailmate HTML messages and a quick test from gmail.

It does not matter what setting I use for "Styling method" (None or Inline). It does not matter if preview is enabled or not.

Moreover, I can't remember if it happened at the same time, but some odd spacing has appeared in the composer as well: All numbers, punctuation marks and spaces seem to have had an extra space added meaning text looks like this: "O n 6 Feb 2 0 1 9 , at 1 9 : 0 4 , ". This happens on all messages, also new ones. So perhaps this should be a separate issue but there could be some relation, hence the mention.

I have not made any special config tweaks, at least not since 2017, so nothing that should suddenly trigger this. I am currently using build 5599.

I have attached a screencast that demonstrates the sluggishness and you can also see the odd spacing problem. The text took just a few seconds to enter, but as you can see it takes a long time to appear.