This has not been true in my experience. I run a Splunk server in production and at my data volumes it has been very performant. It's also much easier to setup and maintain than ELK clusters.

In the early days Splunk pricing was exorbitant (we evaluated Splunk 7 years ago and dismissed it), but licensing has changed in recent years and it is now priced by volume ingested (the pricing is transparent and listed on their website now). At low volumes, the pricing is similar to Sumologic, and is pretty accessible now to smaller dev shops. Open-source collectors like fluentd also help to intelligently reduce the ingest volume.

At high volumes, the TCO changes quite a bit.