Thanks to its wealth of plugins, Jenkins can easily connect many types of systems. Therefore, Jenkins' usage can grow virally in an organization. Testing and Javadoc generation takes up system resources. A master Jenkins is best used to report back quickly on the jobs distributed across a range of Jenkins nodes. This approach makes it easier to analyze where the failure lies in the infrastructure.
If you are using JMeter for your performance tests at scale, consider offloading from Jenkins to a cloud service such as BlazeMeter (http://blazemeter.com/).
For functional testing with Selenium, there is also a wide range of cloud services. Consider using them not only because of load, but also because of the use of a wide range of browser types and versions offered. One example of a commercial service is Sauce Labs (https://saucelabs.com/). It...