MongoDB has a default limit of 64k incoming connections, and creating a thread per connection can lead to performance issues on Linux servers due to limited ulimits such as nofile, nproc, and stack. Increasing these limits using the `ulimit` command and adjusting kernel settings like `max_map_count` and `ip_local_port_range` can help create tens of thousands of connections. The author found that EC2 instances from certain instance types (M5) have a lower limit on incoming connections compared to others (c3), highlighting the importance of considering instance type when tuning for high-performance connections.