open mpi starts very fast but slows down massively soon after - c

Question

I have a C program that takes a very large file (can be 5GB to 65GB) and transposes the data in the file and then writes out the transposed data to other files. In total, the results files are approx 30 times larger due to the transformation. I am using open mpi so each processor used writes to it's own file.
Each processor writes the first ~18 GB of data to it's own results file at a very fast speed. However, at this stage the program slows to a crawl and the %CPU on the top command output drops drastically from ~100% to 0.3%.
Can anyone suggest a reason for this? Am I reaching some system limit?
Code:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
unsigned long long impute_len=0;
void write_results(unsigned long long, unsigned long long, int);
void main(int argc, char **argv){
// the impute output
impute_fp=fopen("infile.txt", "r");
// find input file length
fseek(impute_fp, 0, SEEK_END);
impute_len=ftell(impute_fp);
//mpi magic - hopefully!
MPI_Status status;
unsigned long long proc_id, ierr, num_procs, tot_recs, recs_per_proc,
root_recs, start_byte, end_byte, start_recv, end_recv;
// Now replicte this process to create parallel processes.
ierr = MPI_Init(&argc, &argv);
//find out process ID, and how many processes were started.
ierr = MPI_Comm_rank(MPI_COMM_WORLD, &proc_id);
ierr = MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
if(proc_id == 0){
tot_recs = impute_len/54577; //54577 is length of each line
recs_per_proc = tot_recs/num_procs;
if(tot_recs % num_procs != 0){
recs_per_proc=recs_per_proc+1;
root_recs = tot_recs-(recs_per_proc*(num_procs-1));
}else{
root_recs = recs_per_proc;
}
//distribute a portion to each child process
int z=0;
for(int x=1; x<num_procs; x++){
start_byte = ((root_recs*54577))+(z*(recs_per_proc*54577));
end_byte = ((root_recs*54577))+((z+1)*(recs_per_proc*54577));
ierr = MPI_Send(&start_byte, 1 , MPI_UNSIGNED_LONG_LONG, x, 0, MPI_COMM_WORLD);
ierr = MPI_Send(&end_byte, 1 , MPI_UNSIGNED_LONG_LONG, x, 0, MPI_COMM_WORLD);
z++;
}
//root proc bit of work
write_results(0, (root_recs*54577), proc_id);
}else{
//must be a slave process
ierr = MPI_Recv(&start_recv, 1, MPI_UNSIGNED_LONG_LONG, 0, 0, MPI_COMM_WORLD, &status);
ierr = MPI_Recv(&end_recv, 1, MPI_UNSIGNED_LONG_LONG, 0, 0, MPI_COMM_WORLD, &status);
//Write my portion of file
write_results(start_recv, end_recv, proc_id);
}
ierr = MPI_Finalize();
fclose(impute_fp);
}
void write_results(unsigned long long start, unsigned long long end, int proc_id){
**logic to write out transposed data here
}
fclose(results_fp);
}