Does each sample represent a unique condition? If they are really representatives of different groups you should use the: --labels flag and compare the conditions rather than compare all samples vs. all other samples.
Your command line should end up looking something like this:
cuffdiff --labels CondA,CondB [all other options] <transcripts.gtf> sample1_from_condA.bam, sample2_from_condA.bam sample3_from_condB.bam, sample4_from_condB.bam
This should reduce the number of comparisons that has to be performed, which should also reduce the run time. As the number of comparisons grows quadratically with the number of conditions to be compared it pays off to construct your study in a reasonable way with respect to this.