Can a singleton service (single instance) be high-load?

Yes, a singleton service (one instance) can be high-load. High load is determined by the number of requests, the volume of data processed, or the intensity of computations, not by the number of instances.

For example, a monolithic application running on a single server can handle thousands of requests per second if hardware resources and optimization allow.

However, a single instance has limitations in scalability and fault tolerance. Under high load, bottlenecks may occur — CPU, memory, I/O, network. To handle increased load, scaling is usually applied:

• Vertical (increasing server resources)
• Horizontal (adding instances)

But initially, a singleton service can be quite high-load if the architecture and infrastructure support it.