Proxycurl scrapes public Linkedin profiles in real-time and on-demand. Web scraping takes time, and on average. Every API Request dispatches a real-time job to scrape a profile. Because each API request is a job to scrape a profile live, it takes about 5-6 seconds to complete and the API to return results.
If profile freshness is of a lower priority for you, you can add `use-cache=if-present` to the request parameter. This will return cached profiles immediately.