The Task Manager is the best tool to use first for assessing running processes and their resource usage on a Windows 10 computer. It provides detailed information about the performance of the system, including CPU, memory, disk, and network usage by each process, which can help to quickly identify applications that are consuming a significant amount of resources and might be causing the computer to slow down.

While the other options like Resource Monitor and System Configuration can also provide valuable information, they serve slightly different purposes. Resource Monitor offers more detailed resource usage data across the system but is a more advanced tool that might not be necessary for an initial assessment. System Configuration is typically used for modifying startup programs and services, not real-time process management. Performance Monitor is also a more advanced tool used for in-depth analysis over time rather than immediate troubleshooting.