[prophesi]

That's an issue with or without client-side encryption. Even with IMAP, you have to download the message before its contents can be searched. While subject/sender/recipient can be searched instantly with IMAP, regardless of encryption.

[pseudalopex]

This is a limitation of your client or server. Not IMAP.

[prophesi]

No, it's a limitation by IMAP that is rectified by the server/client.

[pseudalopex]

IMAP's SEARCH TEXT command is at least 35 years old.[1]

[1] https://datatracker.ietf.org/doc/html/rfc1064