I may not be understanding, but I'll try: local folders are stored locally on your computer, not on the server.

I want to know if they are in memory, thus slowing down operations, or merely sitting on the disk for access when required. In the bigger picture, how much is put in memory? Are all mails only loaded when accessed? What about newly arrived mails? Thanks for responding. I am just wondering whether I should archive last quantities of emails to speed up the program

Any email stored in the 'Local Folders' mail account is only stored on your computer and it will be stored in mbox files.

Any email stored in a Pop account is usually fully downloaded and therefore stored locally on your computer in mbox files.

Any email in an Imap account might be full downloaded or only headers downloaded and this can apply on a per folder basis and also on a set number of emails dependent upon date basis. These settings are set up in the Account Settings > synchronisation & storage. If emails are fully downloaded then you would consider those to be a local copy stored in an mbox file, but as imap folders synchronise with server, what is stored locally depends upon what is stored on server.

Any email stored 'locally' is stored in an mbox file. eg: the 'Inbox' is a single mbox text file which may have several emails in it, written one after the other in order downloaded.

If only headers are downloaded then there is no mbox file and emails are not stored locally on computer.

All headers for all emails are stored in appropriate 'Index' files and the they have same name as mbox file but have a .msf extension eg: Inbox.msf These index files are considerably much smaller and use minimal memory.

Thunderbird uses the index files when it displays emails in the 'Message List' aka Thread Pane. It also uses index file when searching etc.

When you select to open and read an email then Thunderbird uses header info to open relevant mbox file, retrieve the email from the mbox file and displays the contents in the Message Pane or tab. At this point it will use more memory as it needs to open the relevant mbox file (assuming email is fully downloaded) to locate the relevant email. The number and size of emails stored in that single file determines how much memory is used.

If you have only downloaded headers in order to save space on disk then communication occurs between Thunderbird and server to download the relevant email into a temp cache in order to open email and display contents. Memory will increase during this process.

Ways to speed up processes: Organise emails into folders to aid with locating emails and reduce file size. Compact folders on a regular basis to remove all old traces of moved or deleted emails. It helps to maintain a healthy file and reduces file size. Move old emails that you need to keep into 'Archive' folders which you set up on a per year structure and also retain folder structure. If you use an Imap account then you could copy emails into the 'Local Folders' mail account setting up various Folders to organise them. I advise 'copy' because if something goes wrong you can repeat the operation without loss. Remember a 'move' means move out of imap and therefore off server, so use Copy and later use the delete to remove from imap folder.

Remember to compact folder after moving or deleting a load of emails.

If you have an Anti-Virus program scanning any file that opened - it will intervene in the process and cause some delay as Thunderbird temporarilly cannot access it's own files then I advise you set up to exclude the Thunderbird profile folder (and all contents therein) from scanning. If profile is in default location then on a Windows OS this the 'Thunderbird' folder you should exclude: C://Users/user name/Appdata/Roaming/Thunderbird

If you do want scanning then please do not allow the AV to 'fix' things because it does not understand that one file may have lots of emails and you could loose a load of data.

Also advise you exclude same Thunderbird file from any cleanup software like CCleaner or Wisecleaner as they have a habit of causing all sorts of inconvenient issues.

Memory means - memory used on disk to store files eg: mbox and index files and RAM memory used to process an action like opening a file. So having a lot of memory used in disk to store stuff can be ok, but having very large files that need to be opened uses RAM and that maybe the slow down you experience.

I think the slowness may be being caused by your anti-virus / anti-malware when it scans your Thunderbird email folder/s before proceeding. I eventually found and followed the instructions here: [https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Relate...] and it finally solved it for me.

There are example settings for several AV products including MS Defender that many people use. You should also check that all your incoming and outgoing email server settings are exactly correct. Mine weren't quite right (I had selected SSL/TLS instead of STARTTLS) and correcting them all made a worthwhile speed difference, but following the info in the link above had the biggest beneficial effect by far, restoring my email sending and download speeds.

Hope this helps you.

Thanks for your time and interesting information and suggestions. I solved the problem by acquiring a new computer with faster processor (i5 to i7) and more RAM

The only anti-virus software I use is that included in Windows