Best way to save lots of streaming data from iPhone - ios

Question

Currently, I am streaming image and depth data from my iphone X and doing a series of computations on it using a compute metal kernel. The output every cycle (frame) is approximately 900,000 floats. Assuming there are 30 cycles (frames) a second, this represents 27million floats a second.
I would like to save a few seconds (3-5) of this data to a file, which I can then access later.
My current approach of writing the output every cycle is too slow (I/O is expensive) and I am not sure how to buffer the results of a few cycles correctly (I tried creating an array of arrays but then run into memory issues).
Copy data from mtl buffers:
//number of elements per frame approx 300,000
size_t len = elementCount
finalArrayX = new float [len];
finalArrayY = new float [len];
finalArrayZ = new float [len];
//MTLBuffers
finalArrayX = (float*) resultsBufferX.contents;
finalArrayY = (float*) resultsBufferY.contents;
finalArrayZ = (float*) resultsBufferZ.contents;
writeToFile(finalArrayX, finalArrayY, finalArrayZ)
writeToFile:
/*
* Get file handle to append to
*/
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *documentTXTPath = [documentsDirectory stringByAppendingPathComponent:#"test.txt"];
if(![[NSFileManager defaultManager] fileExistsAtPath: documentTXTPath]) {
[[NSFileManager defaultManager] createFileAtPath: documentTXTPath contents:nil attributes:nil];
}
NSFileHandle *myHandle = [NSFileHandle fileHandleForWritingAtPath:documentTXTPath];
[myHandle seekToEndOfFile];
/*
* Create strings and write line by line
*/
for(int k = 0; k < elementCount; k ++){
NSString *x_i = [NSString stringWithFormat:#"%.2f", finalArrayX[k]];
NSString *y_i = [NSString stringWithFormat:#"%.2f", finalArrayY[k]];
NSString *z_i = [NSString stringWithFormat:#"%.2f", finalArrayZ[k]];
NSString *temp = [line stringByAppendingString: x_i];
temp = [temp stringByAppendingString: #" "];
NSString *tempTwo = [temp stringByAppendingString: y_i];
tempTwo = [tempTwo stringByAppendingString: #" "];
NSString *tempThree = [tempTwo stringByAppendingString: z_i];
NSString *totalLine = [tempThree stringByAppendingString: #"\n"];
[myHandle writeData:[totalLine dataUsingEncoding:NSUTF8StringEncoding]];
}
Whats the best way to go about this?