Describe the bug
According to yourkit [1] we have a lot of blocking happening in this line (almost all red blocks are related to this line):

overallResult.complete(new ExecutionResultImpl(builder.build(), executionContext.getErrors()));

as ExecutionContext defines it as synchronizedList:

    private final List<GraphQLError> errors = Collections.synchronizedList(new ArrayList<>());

    /**
     * @return the total list of errors for this execution context
     */
    public List<GraphQLError> getErrors() {
        return ImmutableList.copyOf(errors);
    }

[1]

To Reproduce
I don't have reproducer but, right now we have a workaround to get the "errors" via reflection, that delivers a massive improvement.

@andimarek @bbakerman IIRC we already discussed this once