At this point of development, it’s the client CPU speed - however many transaction signatures their computer can manage (which is actually quite high). The limit is not the network, which can process updates to coin data extremely quickly.
I did a test of this back in the days of Structured Data.
the rate was usually between 120 - 130 updates per second
CPU load across all vaults was negligible.
CPU load on the client was 100% of one core
I don’t have any statement to make on the fee situation or network congestion until there’s more clarity around the actual design and implementation of safecoin. I do think some incentive is needed to mitigate spam, but I consider that to be a general problem, not a safecoin problem.