Performing any long running task or CPU intensive work on transport_worker threads is problematic. This is typically very bad for cluster stability.

This issue has been filed to examine the usage of transport_worked threads by ESQL, with the intention of "offloading" as much as possible, including deserialising large(ish) amounts of data.
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/enrich/EnrichLookupService.java
private final Page page;
        LookupResponse(Page page) {
            this.page = page;
        }
        LookupResponse(StreamInput in) throws IOException {
            this.page = new Page(in);
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            page.writeTo(out);
        }
    }
}