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
this.shardId = new ShardId(in);
            this.matchType = in.readString();
            this.matchField = in.readString();
            this.inputPage = new Page(in);
            PlanStreamInput planIn = new PlanStreamInput(in, PlanNameRegistry.INSTANCE, in.namedWriteableRegistry(), null);
            this.extractFields = planIn.readList(readerFromPlanReader(PlanStreamInput::readNamedExpression));
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            super.writeTo(out);
            out.writeString(sessionId);
            out.writeWriteable(shardId);
            out.writeString(matchType);
            out.writeString(matchField);
            out.writeWriteable(inputPage);
            PlanStreamOutput planOut = new PlanStreamOutput(out, PlanNameRegistry.INSTANCE);
            planOut.writeCollection(extractFields, writerFromPlanWriter(PlanStreamOutput::writeNamedExpression));
        }
        @Override
        public String[] indices() {
            return new String[] { shardId.getIndexName() };
        }
        @Override
        public IndicesOptions indicesOptions() {
            return IndicesOptions.strictSingleIndexNoExpandForbidClosed();
        }
        @Override
        public Task createTask(long id, String type, String action, TaskId parentTaskId, Map<String, String> headers) {
            return new CancellableTask(id, type, action, "", parentTaskId, headers) {
                @Override