Benchmark to measure how much slower property access is vs direct member access.

property-access.m

// Compile as: clang -O4 -framework Foundation property-access.m && ./a.out

#import <Foundation/Foundation.h>

// Performance timer, credit: Lars Schneider @kit3bus
#import <mach/mach_time.h>

#define MAKE_NSSTRING(str) (NSString *)CFSTR(#str)

#define START(name) \
uint64_t name##_startTime = 0;\
uint64_t name##_endTime = 0;\
uint64_t name##_elapsedTime = 0;\
uint64_t name##_elapsedTimeNano = 0;\
mach_timebase_info_data_t name##_timeBaseInfo;\
mach_timebase_info(&name##_timeBaseInfo);\
name##_startTime = mach_absolute_time();\

#define STOP(name) \
name##_endTime = mach_absolute_time();\
name##_elapsedTime = name##_endTime - name##_startTime;\
name##_elapsedTimeNano = name##_elapsedTime * name##_timeBaseInfo.numer / name##_timeBaseInfo.denom;\
NSLog(@"Time %@ %"PRIu64"ms", MAKE_NSSTRING(name), name##_elapsedTimeNano / 1000000);\


@interface MyObject : NSObject
{
@public
	NSString * _string;
}
@property (retain) NSString *string;
@end

@implementation MyObject
@synthesize string = _string;
@end


int main (int argc, char const *argv[])
{
	size_t times = 1000000;
	
	MyObject * obj = [[MyObject alloc] init];
	
	START(property_access);
	for(size_t i = 0; i < times; ++i)
		//[obj setString:@"FOO bar"];
		obj.string = @"FOO bar";
	STOP(property_access);
	
	START(ivar_access);
	for(size_t i = 0; i < times; ++i)
		obj->_string = @"FOO bar";
	STOP(ivar_access);
	
	[obj release];
	
	return 0;
}

results.txt

Time property_access 8ms
Time ivar_access 0ms