A simple benchmarking experiment to compare enumeration techniques in Objective-C. My results are in the comments.

enumeration-benchmark.m

#import <Foundation/Foundation.h>

void timeThis(NSString *label, NSUInteger iterations, void(^block)()) {
    
    CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
    for (NSUInteger i = 0; i < iterations; i++) {
        block();
    }
    CFAbsoluteTime interval = CFAbsoluteTimeGetCurrent() - start;
    printf("%s: %lu iterations, %.3fs\n", [label UTF8String], iterations, interval);
    
}

int main(int argc, const char * argv[]) {

    @autoreleasepool {
        
        NSUInteger numberOfIterations = 20000;
        NSUInteger numberOfElements = 5000;
        
        NSMutableArray *strings = [NSMutableArray arrayWithCapacity:numberOfElements];
        for (NSUInteger i = 0; i < numberOfElements; i++) {
            [strings addObject:[NSMutableString stringWithFormat:@"Object %lu", i]];
        }

        timeThis(@"Fast enumeration", numberOfIterations, ^{
            for (NSMutableString *string in strings) {
                [string appendString:@"."];
            }
        });

        timeThis(@"Block", numberOfIterations, ^{
            [strings enumerateObjectsUsingBlock:^(NSMutableString *string, NSUInteger idx, BOOL *stop) {
                [string appendString:@"."];
            }];
        });

        timeThis(@"Block, concurrent", numberOfIterations, ^{
            [strings enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(NSMutableString *string, NSUInteger idx, BOOL *stop) {
                [string appendString:@"."];
            }];
        });

    }
    return 0;
}

My results (MacBook Pro 2.6GHz i7):

Fast enumeration: 20000 iterations, 5.724s
Block: 20000 iterations, 10.921s
Block, concurrent: 20000 iterations, 3.324s