Typing in Messages is 'laggy' since installing Mac OS Sequoia 15.1 update

This issue exists here on a 2019 Macbook Pro 16" and a 2020 Macbook Pro 13", both with MacOS 15.1 (24B83).

Here is what I have observed:

• iMessage text entry becomes increasingly slow as longer messages are entered.
• Short messages do not seem to be affected. By about the 800 character mark, the text entry box is laggy, sometimes so much as to be unusable.
• The slow text entry problem does not affect any other software which accepts text entry - it is specific to iMessage, as far as I can tell. (I do not use Whatsapp.)
• This is not related to any third party security software (I have none).
• The problem occurs regardless of whether any other software is running (I have reproduced this on a clean reboot).
• I have removed all peripherals except my USB keyboard, so there is no relationship to (for example) my external GPU.
• There are two third-party drivers installed: OpenVPN, and a Canon printer driver.
• This problem was not reproducible on my 2020 Macbook Pro 13" with MacOS 15.0.1. It was immediately reproducible after I upgraded it to 15.1, so that seems pretty definitive.
• So far, I cannot reproduce this problem on an Apple M1 Pro 16" with MacOS 15.1, though I see that others have reproduced it on this platform. On this particular M1, Apple Intelligence is offered only as a waitlist item, not yet an available feature, in case that is significant.

I have not found any reliable way to prevent this from occurring when I experience it, so I have resorted to composing longer messages using vi in a terminal and then pasting them into iMessage.