Advice on overcoming a WebService Callout time out error using MetadataAPI

My solution was first to store custom Metadata. With each row containing

• Object
• RecordType
• Picklists ('Field_1__c, Field_2__c, Field_3__c')

Then going forwards-

1. Integration team call to a WebService Apex class
2. That WebService Apex class starts a batch Job
3. The batch Job fires an Apex Class

The below is the method that is commonly timing out. Both 'service.ReadMetadata' are in the same method doing callouts. So I'm not sure how to synchronously fix these to retain within 120s.

//List<String> recordType = List of picklist fields available in the recordType
//DescribeSObjectResult objectResult = Current object information
//Set<String> metadataFields = Set of fields to udpate listed in the custom metadata as above
//String recordTypeName = recordTypeName label
public Set<String> processPicklist(List<String> recordType, DescribeSObjectResult objectResult, Set<String> metadataFields, String recordTypeName ) { 
        for(String picklistValue : recordType) {
            if(metadataFields.contains(picklistValue)){
                String ctObjectRecordType = String.valueOf(''+objectResult.getLocalName()+'.'+recordTypeName+'');
                String fieldName = String.valueOf(''+objectResult.getLocalName()+'.'+picklistValue+'');
                List<MetadataService.PicklistValue> pickVals = new List<MetadataService.PicklistValue>();
// Either of these two lines are timing out when it comes to error logs
                MetadataService.CustomField  picklistFieldMdApi = (MetadataService.CustomField)   service.readMetadata('CustomField', new String[] { fieldName }).getRecords()[0];
                MetadataService.GlobalValueSet  globalPickListApi = (MetadataService.GlobalValueSet) service.readMetadata('GlobalValueSet', new String[] { picklistFieldMdApi.valueSet.valueSetName }).getRecords()[0];
                for(MetadataService.CustomValue existing : globalPickListApi.customValue){
                    MetadataService.PickListValue newPickVal = new MetadataService.PickListValue();
                    newPickVal.fullName = existing.fullName;
                    newPickVal.label = existing.label;
                    newPickVal.isActive = true;
                    newPickVal.default_x = existing.default_x;
                    pickVals.add(newPickVal);
                }
                MetadataService.Metadata[] mdRecordTypes = service.readMetadata('RecordType', new String[]{ctObjectRecordType}).getRecords();
                MetadataService.RecordType[] recordTypes = (MetadataService.RecordType[]) mdRecordTypes;} else {recordTypes = new MetadataService.RecordType[]{};
                MetadataService.RecordTypePicklistValue[] rtPickValues = new MetadataService.RecordTypePicklistValue[]{};
                MetadataService.RecordTypePicklistValue rtPickValue = new MetadataService.RecordTypePicklistValue();
                rtPickValue.picklist = picklistValue;
                rtPickValue.values = pickVals;
                rtPickValues.add(rtPickValue);
                for(MetadataService.RecordType RT : recordTypes){ RT.PicklistValues = rtPickValues;}
                List<MetadataService.SaveResult> results = new List<MetadataService.SaveResult>();
                results = service.updateMetadata(new MetadataService.Metadata[] { mdRecordTypes[0]  }); 
            }
        }
        return metadataFields;
    }