I have a Gmail account since it was invite only and I believe also there wasn't any web interface yet so I always used it through a mail client such as Sylpheed, now Claws Mail (
https://www.claws-mail.org/). The very few times i had to use the web interface (always Firefox or derivatives) it worked without hitches on FF too, but a few months ago I was doing some maintenance to a customer PC (Debian+Firefox just like all my machines) and noticed how slow the Gmail web interface had become. Switching to the "old" mode helped somewhat to restore some speed, but being used to snappy dedicated mail client it still was truly slow to operate. Unfortunately today every non tech savy user thinks
the browser is the internet to the point some don't even know what the word browser mean, which makes even more difficult to convince them to try anything else.
Where a browser struggles with two pages of mails, I can keep available my entire archive since about 2001 on the Claws Mail client, that's over 50K mails in a dozen accounts, and query them to find whatever I need.