The last of the issues mentioned earlier that you may experience when dealing with such types of problems are rate limits that have been imposed by external service providers. In the case of the foreign exchange rates API, the service maintainer did not inform us about any rate limits or throttling mechanisms. But many services (even paid ones) often do impose rate limits. Also, it isn't fair to abuse a service that is provided to users completely for free.
When using multiple threads, it is very easy to exhaust any rate limit or simply—if the service does not throttle incoming requests—saturate the service to the level that it will not be able to respond to anyone (this is known as a denial of service ...
