Hi @adam@act360.ca,
Thank you for sharing your question with the Klaviyo Community.
I noticed that a member of our staff was able to address your question and so I am reposting an edited version of their reply below.
All of our endpoints are rate limited. When you hit rate limits or notice that events are not being posted to Klaviyo , we recommend adjusting your request-rate using an exponential backoff - as you found using the sleep function which allowed all 5 events to come through.
Klaviyo Rate limits are in line with industry standard limits. For example, depending on the endpoint Google rate limits between 2-20 calls per second. Shopify limits all REST API calls to 2-4 per second.
Thanks and have a great day.