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 static Page createNullResponse(int positionCount, List<NamedExpression> extractFields) {
        final Block[] blocks = new Block[extractFields.size()];
        for (int i = 0; i < extractFields.size(); i++) {
            blocks[i] = Block.constantNullBlock(positionCount);
        }
        return new Page(blocks);
    }
    private static Operator droppingBlockOperator(int totalBlocks, int droppingPosition) {
        BitSet bitSet = new BitSet(totalBlocks);
        bitSet.set(0, totalBlocks);
        bitSet.clear(droppingPosition);
        return new ProjectOperator(bitSet);
    }
    private class TransportHandler implements TransportRequestHandler<LookupRequest> {
        @Override
        public void messageReceived(LookupRequest request, TransportChannel channel, Task task) {
            ActionListener<LookupResponse> listener = new ChannelActionListener<>(channel);
            doLookup(
                request.sessionId,
                (CancellableTask) task,
                request.shardId,
                request.matchType,
                request.matchField,
                request.inputPage,
                request.extractFields,
                listener.map(LookupResponse::new)
            );
        }
    }
    private static class LookupRequest extends TransportRequest implements IndicesRequest {