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
import org.elasticsearch.compute.operator.Driver;
import org.elasticsearch.compute.operator.DriverContext;
import org.elasticsearch.compute.operator.Operator;
import org.elasticsearch.compute.operator.OutputOperator;
import org.elasticsearch.compute.operator.ProjectOperator;
import org.elasticsearch.compute.operator.SourceOperator;
import org.elasticsearch.index.mapper.MappedFieldType;
import org.elasticsearch.index.query.SearchExecutionContext;
import org.elasticsearch.index.shard.ShardId;
import org.elasticsearch.search.SearchService;
import org.elasticsearch.search.internal.AliasFilter;
import org.elasticsearch.search.internal.SearchContext;
import org.elasticsearch.search.internal.ShardSearchRequest;
import org.elasticsearch.tasks.CancellableTask;
import org.elasticsearch.tasks.Task;
import org.elasticsearch.tasks.TaskId;
import org.elasticsearch.transport.TransportChannel;
import org.elasticsearch.transport.TransportRequest;
import org.elasticsearch.transport.TransportRequestHandler;
import org.elasticsearch.transport.TransportRequestOptions;
import org.elasticsearch.transport.TransportResponse;
import org.elasticsearch.transport.TransportResponseHandler;
import org.elasticsearch.transport.TransportService;
import org.elasticsearch.xpack.core.ClientHelper;
import org.elasticsearch.xpack.esql.EsqlIllegalArgumentException;
import org.elasticsearch.xpack.esql.action.EsqlQueryAction;
import org.elasticsearch.xpack.esql.io.stream.PlanNameRegistry;
import org.elasticsearch.xpack.esql.io.stream.PlanStreamInput;
import org.elasticsearch.xpack.esql.io.stream.PlanStreamOutput;
import org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner;