Passage: In March 1948 the converter unit was installed, which made possible programming through the reader from standard IBM cards. The "first production run" of the new coding techniques on the Monte Carlo problem followed in April.  After ENIAC's move to Aberdeen, a register panel for memory was also constructed, but it did not work.  A small master control unit to turn the machine on and off was also added.The programming of the stored program for ENIAC was done by Betty Jennings, Clippinger, Adele Goldstine and others. It was first demonstrated as a stored-program computer in April 1948, running a program by Adele Goldstine for John von Neumann. This modification reduced the speed of ENIAC by a factor of 6 and eliminated the ability of parallel computation, but as it also reduced the reprogramming time to hours instead of days, it was considered well worth the loss of performance. Also analysis had shown that due to differences between the electronic speed of computation and the electromechanical speed of input/output, almost any real-world problem was completely I/O bound, even without making use of the original machine's parallelism. Most computations would still be I/O bound, even after the speed reduction imposed by this modification.

Question: What did the modification eliminate the ability of?
parallel computation