Witness servers are producing blocks. Compare this website to Facebook. It deals with enormous amounts of data, still, It's fast and efficient. It's run by witnesses who produce blocks and seed nodes that store data, competing with infrastructure and server farms of facebook for example.

While you can theoretically use Virtual RAM or SSD, you are likely to suffer from missing blocks, and network will suffer the speed it needs.

At the time of writing, physical ram is the only way to go. You can query your node, and check the latency, if your node is able to respond with the low latency (<50ms) using any other technology, you could be also good to go.