IMAP server polling has become increadibly slow

[#1]

The IMAP server polling has been slowing down for a while and has become by far the slowest out of the six IMAP servers I am using all the time.

This makes even the basic webmail interface incredibly slow, but to be clear, I am referring to the IMAP server, which I access directly with different mail clients on different machines typically connected to my router (eg Thunderbird on Windows and Linux over ethernet, K-9 on Android over wifi). In fact, it is so slow that it takes 10 seconds or so to poll the inbox every time I select it, and K-9 often times out. Basic webmail is almost unusable. This is more about polling and the time it takes the server to wake up, rather than transfer rate.  

My IMAP setup is tried and tested over many years, so it's not that something changed here. 

I know Plusnet de-prioritised email many yeas ago, but can someone please let us know what is happening.

[#2]

On the assumption it is the Plusnet email so <name@plusnetusername.plus.com> I've not noticed problems with using it with an old version of Microsoft Outlook.

I would wonder about mailbox size, my Plusnet box is only about 25MB, my GMail is perhaps 2GB and I've others at 5GB.

[#3]

The mailbox size in MBs shouldn't really be an issue when polling which only looks at the index. The number of messages (regardless of volume) seems to be a factor though. I select a mailbox with 200 stored messages but no new messages. The server takes 4-5 seconds to start responding (Thunderbird reports 'opening...'). Presumably then it responds with 'no new messages'. Another mailbox with 1600 messages takes 20+ seconds. Both should take a fraction of a second, like much much larger mailboxes with other providers. In fact, when polling other servers it all happens so fast I can hardly see that 'opening...' that Thunderbird displays. The fact that Plusnet's basic webmail interface takes so long to get a response from the server speaks for itself.