Search has been dramatically improved. Difficulties with large mailboxes are often a complex function of many variables with things like client side javascript decryption speeds often playing a large role. While testing is done with large mailboxes as part of the development process, the ultimate solution for people with extremely large boxes will likely be the use of the Bridge program with an IMAP mail client such as Microsoft Outlook. There are unofficial export programs available that call pull all mail out through Proton's API. An official, supported, export (and import) program is planned for the future.
Unless this was introduced within the last couple of months, I never had the option to do IMAP. I wanted it... I would have happily offloaded all that to a local mail client. Plus I would now possess copies of all my email for the year.
