Windows Live Mail 2012 taking longer to send emails of ~1Mb

When I reply to Gumtree inquiries, the size of the email file approaches 1Mb. Recently, there has been a bottleneck in the Outbox when several such replies are sent in quick succession. This is wholly new - as it worked fine for many years.
I get around the problem by moving the banked up emails to DRAFTS and then re-sending them one by one while monitoring the sending progress. Nowadays I typically get an alert that the server has not responded for 60 sec, and am asked if I want to wait longer. I DO wait, and the email gets sent. How can I restore the former functionality of W.L.M. ?

"you might want to consider moving to a newer email client." Not only did I consider it, but I actually did it, after an earlier crisis. I went to Win10's Mail, but could not get back to WLM quick enough. In the meantime, this new irritant would be great to remedy.

Windows Live Mail 2012 is over 3 years out of support.
I used to use it years ago but it was not that great as 2 out of my 6 Accounts would not use IMAP with it, so even back then not so great.
Now all 6 Mail Accounts work with the Windows 10 Mail App. This is updated and has improved a bit over the years.

Email servers also develop over the years and they won't be made compatible with old unsupported applications. Also spam reducing methods maybe more strict, like sending Emails in quick succession a spam like characteristic.

Well the last comment might be useful - but it is NOT the rapidity of emails that is NEWLY causing a problem, it is when they are about 1MB (and presumably larger as well).
While you apparently appreciate the developments in email since the 2012 version of W.L.M., I DEFINITELY do not share that attitude. As I wrote, I tried Win10 MAIL, and could not get back to WLM quick enough. But I would really like it to return to its former functionality. I am NOT looking for the extras that later programs provide.

I dont understand the question. There is ONE account - mine - and it links to Gmail. I sincerely hope you are right - that there is some setting I can tweak to restore its former reliable service. I'll happily answer any further questions if they are sufficiently clear for me to understand them

I have a Gmail acccount, and can access it as webmail, but prefer to use the W.L.M. client for my email.

Many settings are involved - the best procedure might be to post screen shots of all four 'tabs' in the Properties of WLM's Gmail : General, Servers, Security and Advanced. But I will first get your response to my suggestion.