When I log in into my web vault it says “Update your encryption settings to meet new security recommendations and improve account protection”. I then click on “Update KDF settings” and “Change KDF”. However, the prompt asking me to update my encryption settings remains. Currently, KDF iterations is set to 100,000.
I believe the recommended number of iterations is 600,000. Once you click on update and change, you’ll need to click IN the field box (that currently shows 100,000) and input the new number. They also recommend changing it in small increments in case there are issues with the higher iteration number, 100,000 at a time. I just input 600,000 and saved it. No issues (on the web app at least, my phone is another matter) Hope this helps 
As @AbberantSalience mentioned, if you use a number less than 600,000 (as recommended), the prompt will remain.
It is recommended to backup your vault before changing your KDF configuration.
While you are at it, you may want to consider changing the KDF algorithm to Argon2id. The default parameters provide stronger protection than 600,000 PBKDF2 iterations, and you may get the additional protection without any performance loss.
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Argon2id is very snappy on my devices. Much improved security against brute force type attacks.
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