Hi oshvartz,

the <max_delay_millis> is a overall parameter that specifies the longest delay that the server is allowed to apply to outgoing updates in order to collect more updates in the same packet. But it is not the right parameter to look at for your specific need.
Limiting the send rate mainly depends on the subscription mode:

a) In RAW mode there is no way to limit the update frequency (but server can always decide to further restrict in case of resources limitations)

b) In DISTINCT and MERGE you can pose a rate for the whole subscription, by specifying a requested max frequency. Here you can find more details for the JavaScript case. But if your purpose is to limit the rate for some specific items, even in the context of the same subscription, you need to override the getAllowedMaxFrequencyItem method of the concrete implementation of your MetadataAdapter.

c) In Command MODE, all the considerations above apply to the update for each single key

You can get more information on this topic by looking at Section 3.2 of the General Concepts document

Regards,
Gianluca