From src/drumkitloasder.cc:

AudioFile *audiofile = load_queue.front();
load_queue.pop_front();
filename = audiofile->filename;
int preload_size = framesize * CHUNK_MULTIPLIER + framesize;
if(preload_size < 1024)
{
	preload_size = 1024;
}
 
// Note: Remove this line to enable diskstreaming
preload_size = ALL_SAMPLES;
 
audiofile->load(preload_size);

CHUNK_MULTIPLIER is a #define in src/audiocache.h:

#define CHUNK_MULTIPLIER 16