If you have more than 60 users signing up per minute, and you are making 1 request to the CMS API for each user, then yes. However there are some major inconsistencies with the API as documented in this post.
My suggestion is to not rely on the CMS API for this. Just store your users somewhere else that is more performant and sync the data back to Webflow CMS when real time performance doesn’t matter as much.
IMHO the CMS API is not fit for production.