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/plugin/TransportEsqlQueryAction.java
request.pragmas(),
            EsqlPlugin.QUERY_RESULT_TRUNCATION_MAX_SIZE.get(settings)
        );
        String sessionId = sessionID(task);
        planExecutor.esql(request, sessionId, configuration, wrap(r -> {
            computeService.execute(sessionId, (CancellableTask) task, r, configuration, listener.map(pages -> {
                List<ColumnInfo> columns = r.output()
                    .stream()
                    .map(c -> new ColumnInfo(c.qualifiedName(), EsqlDataTypes.outputType(c.dataType())))
                    .toList();
                return new EsqlQueryResponse(columns, pages, request.columnar());
            }));
        }, listener::onFailure));
    }
    /**
     * Returns the ID for this compute session. The ID is unique within the cluster, and is used
     * to identify the compute-session across nodes. The ID is just the TaskID of the task that
     * initiated the session.
     */
    final String sessionID(Task task) {
        return new TaskId(clusterService.localNode().getId(), task.getId()).toString();
    }
    public ExchangeService exchangeService() {
        return exchangeService;
    }
    public EnrichLookupService enrichLookupService() {
        return enrichLookupService;
    }
}