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
public static final String LOOKUP_ACTION_NAME = EsqlQueryAction.NAME + "/lookup";
    private final ClusterService clusterService;
    private final SearchService searchService;
    private final TransportService transportService;
    private final Executor executor;
    public EnrichLookupService(ClusterService clusterService, SearchService searchService, TransportService transportService) {
        this.clusterService = clusterService;
        this.searchService = searchService;
        this.transportService = transportService;
        this.executor = transportService.getThreadPool().executor(EsqlPlugin.ESQL_THREAD_POOL_NAME);
        transportService.registerRequestHandler(
            LOOKUP_ACTION_NAME,
            EsqlPlugin.ESQL_THREAD_POOL_NAME,
            LookupRequest::new,
            new TransportHandler()
        );
    }