Performance will probably suffer the most regarding the amount of time it takes for searches to return results. Remember that one of the main features of Elasticsearch is how fast it can retrieve data and display it.
Having an undersized CPU configuration will lead to searches taking longer than expected, which can result in a frustrating user experience.
In the following screenshot, we can see that search latency spikes to almost 80 ms and hovers at around 20 ms:
Monitoring latency in Kibana
Note that the preceding screenshot was taken from an undersized system with just one CPU running at less than 2 GHz. The latency could be worse, but this was taken from a system running on a fast NVMe drive, which can have latency as low as 100 microseconds.