Performance/Load test using selenium webdriver prerecorded steps
Given you have tests written in C# the most obvious way would be using Visual Studio Load Testing capabilities.
If you are looking for a free and open-source solution I would recommend going for Apache JMeter. JMeter has integration with Selenium via WebDriver Sampler plugin so you should be able to run your Selenium tests in multi-threaded manner. However you will need to convert your C# code into one of the WebDriver Sampler supported languages (default is JavaScript)
Remember that Selenium tests are very resource-intensive as real browsers consume a lot of CPU/RAM so the number of virtual users you will be able to mimic this way will be very limited. So recommended approach is creating main load on a HTTP protocol level and use one Selenium instance to check rendering speed while your application is under the load.
You can install WebDriver Sampler plugin using JMeter Plugins Manager.
Why do you need to run full browsers. HTTP layer tests are far simpler? You also should be looking at only a subset of business processes in performance which generate a preponderance of load.