Very Slow to upload emails from the server

Since the latest update, the uploading of new emails from the server can take minutes to hours. I know the emails are on the server, because I can see them by logging onto my email client on my iPad. Why has Thunderbird suddenly become very frustrating to use? An example which just happened - I was required to enter a 6 digit code to be able to confirm my account before I could submit this question. I waited, and waited and waited to see this code in my Inbox. Finally I went to my iPad to get the code, so I could submit this question. It was there as I expected, but not in my Thunderbird Inbox for another 10 minutes. This is happening far too often, so I am contemplating finding another email program which is not so frustrating.