Yes, buffer size can have a significant effect on performance. You can quickly see buffer sizes used by "read" across your system with "dtrace -n 'syscall::read:entry{ @ = quantize(arg2); }'", which summarizes the output (in case you're doing more of these than you can reasonably see in the console) and has significantly less impact on the program you're tracing. Output for my system:

           value  ------------- Distribution ------------- count    
               0 |                                         0        
               1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  1278     
               2 |                                         0        
               4 |                                         0        
               8 |                                         0        
              16 |                                         0        
              32 |                                         0        
              64 |                                         0        
             128 |                                         0        
             256 |                                         0        
             512 |                                         1        
            1024 |                                         0        
            2048 |@                                        20       
            4096 |                                         2        
            8192 |                                         0        
           16384 |                                         0        
           32768 |                                         0        
           65536 |                                         3        
          131072 |                                         2        
          262144 |                                         1        
          524288 |                                         0    

and if you want to know where, say, the 512-byte reads are coming from:

# dtrace -n 'syscall::read:entry/arg2 == 512/{ @[ ustack() ] = count(); }'

  mdworker                                          
              libSystem.B.dylib`read+0xa
              Foundation`-[NSConcreteFileHandle readDataOfLength:]+0x1d6
              RichText`GetMetadataForURL+0x338
              mdworker`0x100006a66
              mdworker`0x100009ec1
              libSystem.B.dylib`_pthread_start+0x14b
              libSystem.B.dylib`thread_start+0xd
                1