Important to add that while this is Blockstack's write scalability bottleneck, Blockstack's read scalability constraint is different and much better than legacy DNS.
Reads are much more common in legacy DNS - orders of magnitudes more people are visiting websites and resolving names to IPs than are updating zonefiles.
In typical usage of Iegacy DNS, end users talk to their ISP's DNS servers which are responsible for answering requests for all of their customers. These ISP DNS servers perform recursive lookups to authoritative name servers for any data that isn't cached locally. The result is that authorative name servers for popular domains are tasked with serving huge numbers of requests. Billions per month or more for some domains. This is why there's a market for managed DNS services that compete on metrics like capacity, resistance to DDOS, uptime and latency.
With Blockstack we deploy a full Blockstack node with the Blockstack Browser. This essentially puts an authoritative DNS server on every device. With Blockstack, we can do authoritative lookups constrainted only by the processing power of the device. These zonefile queries are done entirely on device and never hit the network or another server.
The implications of this are huge. This means you can register a Blockstack name for mere fractions of a bitcoin and I can perform trillions and trillions of lookups on your name's zone file and have no impact on network resources, a service you have to pay for and most importantly your pocketbook.
Blockstack's incredible read scalability - far superior to legacy DNS - delivers better performance and experience to end users: faster, more secure, more reliable lookup at much lower cost to name owners. It makes a number of attacks: DNS poisoning, DDOS of name servers, etc things of the past.
The trade off is that the user needs to spend some of their disk space storing zone files and their CPU time & network bandwidth syncing and verifying those files.