Thunderbird download speed

I use Thunderbird as an email client.
I have noticed a strange thing. My connection speed 10Mbps and it works ok with Firefox and downloading.
But when I download my emails, speed is much slower. Around 2 Mbps, so 5x slower.
With just plain text it does not matter, but when attachments come, then it matters?
I have asked my email provider if they save some restrictions, but he says ‘none’.

Any clue to solve this mystery?

Thunderbird “eats” whatever it gets. At whatever speed it gets it’s food. Meaning this is not Thunderbird. If the rest is up to speed, it must be the connection to the mailserver. Your provider can only report about their own mailservers. Are we talking about a provider mailaddress?