Can WinCC OA be used on a PC with low performance?

Yes. You can run a WinCC OA server on a relatively small device.

Of course the system requirements depend on the performance requirements of the specific project.
For example there are small WinCC OA projects running on a Industrial PC and there are large projects that require systems with multicore processor servers connected to big Oracle Clusters.

A WinCC OA-server would probably need: Data-Manager, Event-Manager, a CTRL Manager running a standard script and possibly a driver. This would require a fairly small footprint.
There may be the same WinCC OA installation on a multi-core server running a large WinCC OA-project containing : N drivers, N CTRL managers, N archive managers, etc..

The point is: There is just one version of WinCC OA suitable for small devices up to large servers.