What is Concurrent Request Limit in Salesforce?

Once a synchronous Apex request runs longer than 5 seconds, it begins counting against this limit. Each organization is allowed 10 concurrent long-running requests. If the limit is reached, any new synchronous Apex request results in a runtime exception. This behavior occurs until the organization’s requests are below the limit.

Some useful tips:
  1. Convert synchronous processes to asynchronous processes. Batch Apex might be a viable alternative. Limit synchronous Web service callouts.
  2. Use the Streaming API instead of polling.
  3. Tune SOQL and DML operations. Make sure that your queries are selective. Limit the number of records in your list views. Avoid data skew.
  4. Use the Continuation class to make callouts asynchronously to a SOAP or REST Web service from a Visualforce page.