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/ComputeService.java
}
    private void runComputeOnRemoteNodes(
        String sessionId,
        CancellableTask rootTask,
        EsqlConfiguration configuration,
        PhysicalPlan dataNodePlan,
        ExchangeSourceHandler exchangeSource,
        List<TargetNode> targetNodes,
        Supplier<ActionListener<DataNodeResponse>> listener
    ) {
        
        final ListenableActionFuture<Void> blockingSinkFuture = new ListenableActionFuture<>();
        exchangeSource.addRemoteSink(
            (sourceFinished, l) -> blockingSinkFuture.addListener(l.map(ignored -> new ExchangeResponse(null, true))),
            1
        );
        try (RefCountingRunnable exchangeRefs = new RefCountingRunnable(() -> blockingSinkFuture.onResponse(null))) {
            
            
            for (TargetNode targetNode : targetNodes) {
                var targetNodeListener = ActionListener.releaseAfter(listener.get(), exchangeRefs.acquire());
                var queryPragmas = configuration.pragmas();
                ExchangeService.openExchange(
                    transportService,
                    targetNode.node(),
                    sessionId,
                    queryPragmas.exchangeBufferSize(),
                    ActionListener.wrap(unused -> {
                        var remoteSink = exchangeService.newRemoteSink(rootTask, sessionId, transportService, targetNode.node);
                        exchangeSource.addRemoteSink(remoteSink, queryPragmas.concurrentExchangeClients());