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
targetNode,
                LOOKUP_ACTION_NAME,
                lookupRequest,
                parentTask,
                TransportRequestOptions.EMPTY,
                new ActionListenerResponseHandler<>(
                    listener.map(r -> r.page),
                    LookupResponse::new,
                    TransportResponseHandler.TRANSPORT_WORKER
                )
            );
        }
    }
    private void doLookup(
        String sessionId,
        CancellableTask task,
        ShardId shardId,
        String matchType,
        String matchField,
        Page inputPage,
        List<NamedExpression> extractFields,
        ActionListener<Page> listener
    ) {
        Block inputBlock = inputPage.getBlock(0);
        if (inputBlock.areAllValuesNull()) {
            listener.onResponse(createNullResponse(inputPage.getPositionCount(), extractFields));
            return;
        }
        try {
            ShardSearchRequest shardSearchRequest = new ShardSearchRequest(shardId, 0, AliasFilter.EMPTY);