Explanation
A single-threaded application is an application that can only execute one task or process at a time. A single-threaded application can only utilize one CPU core, regardless of how many cores are available or assigned to the virtual machine. Therefore, adding more vCPU cores to the VM will not improve the performance of the application, as it will still be limited by the speed and capacity of one core12.
To troubleshoot this issue, the administrator should check if the application is single-threaded or multi-threaded. This can be done by using tools such as Task Manager, Performance Monitor, or Process Explorer on Windows, or top, htop, or ps on Linux34. If the application is single-threaded, the administrator should consider the following options:
Reduce the number of vCPU cores on the VM to match the number of threads that the application can use. This can help avoid CPU contention and co-stop issues that may arise from having too many vCPUs relative to the number of physical cores on the host5.
Upgrade the physical CPU on the host to a faster or newer model that can provide higher clock speed and performance for the single core that the application uses.
Optimize the application code or configuration to make it more efficient or multi-threaded, if possible.
This can help the application take advantage of multiple cores and improve its performance.

1U stands for one unit and it is a standard unit of measurement for rack-mounted servers. It is equal to 1.75 inches (4.45 cm) in height. A 42U rack can accommodate 42 1U servers or a combination of servers with different unit sizes. Therefore, the unit size per server should be 1U if there are 19 1U servers in a 42U rack. Reference: https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam-objectives (Objective 1.2)

Active-active is a configuration in which both nodes of a redundant system respond to service requests whenever possible. It can improve the performance, availability, and load balancing of the system by distributing the workload among the nodes. However, it also requires more synchronization and coordination between the nodes to avoid conflicts or errors. Verified Reference: [Active-active], [Redundant system]