Four different Gaussian jobs were run on four different computers:

CLUSTER (hereafter designated as SCH)
the cluster (schroedinger.cem.msu.edu)

Linux PC (hereafter designated as PC)
Linux PC (Red Hat 7.3; Intel Pentium 4; clock 1.7 GHz; 256 KB cache memory; 512 MB memory; (nash.bch.msu.edu))

SGI (hereafter designated as SGI-N (non-dedicated CPUs) and SGI-D (dedicated CPUs))
SGI computer (16 Processors; clock 300 MHz;
CPU: MIPS R12000 Processor Chip Revision: 2.3;
FPU: MIPS R12010 Floating Point Chip Revision: 2.3; (mcbisgi.cem.msu.edu))

Two processor Linux PC (hereafter designated as 2CPU-HT (with Hyper-Threading) and 2CPU-NHT (without Hyper-Threading))
Two processor Linux PC (Red Hat 7.3; Intel Xeon, clock 2.66 GHz, 512 KB cache memory; 1 GB memory; two processors with Hyper-Threading Technology; four virtual processors (helios.bch.msu.edu))

The jobs were as follows:

QCISD
Geometry optimization of gauche as well as anti conformers of ethanol at
qcisd(full)/6-31++G**//qcisd(full)/6-31++G**

CCSD(T)
Single point calculation of gauche as well as anti conformers of ethanol at
ccsd(t)(full)/6-311++G**//qcisd(full)/6-31++G**

DFT OPT
Geometry optimization of an organic molecule (C1 symmetry) at
b3lyp/6-311++g**// b3lyp/6-311++g**

DFT FREQ
Calculation of vibrational frequencies of the same organic molecule at
b3lyp/6-311++g**// b3lyp/6-311++g**
employing the optimized geometry

The results: times (in minutes) and parallelization per processor (in %)
("50%" means no parallelization; "100%" means full parallelization)

COMPUTER	SCH	PC	2CPU-HT	2CPU-NHT	SGI-N	SGI-D
JOB
SPACE
QCISD gauche EtOH (C1) %NPROC=1	145	117	83	83	414	431
SPACE
QCISD gauche EtOH (C1) %NPROC=2	145 (50%)	-	83 (50 %)	83 (50%)	351 (59%)	385 (56%)
SPACE
QCISD anti EtOH (Cs) %NPROC=1	52	39	29	29	132	128
SPACE
QCISD anti EtOH (Cs) %NPROC=2	51 (51%)	-	29 (50 %)	29 (50%)	126 (52%)	118 (54%)
SPACE
CCSD(T) gauche EtOH (C1) %NPROC=1	45	41	26	26	102	111
SPACE
CCSD(T) gauche EtOH (C1) %NPROC=2	44 (50%)	-	26 (50%)	26 (50%)	82 (62%)	79 (70%)
SPACE
CCSD(T) anti EtOH (Cs) %NPROC=1	19	16	11	11	45	43
SPACE
CCSD(T) anti EtOH (Cs) %NPROC=2	19 (50%)	-	11	11 (50%)	43 (52%)	43 (50%)
SPACE
DFT OPT %NPROC=1	1755	1519	949	963	2461	2542
SPACE
DFT OPT %NPROC=2	998 (88%)	-	617 (77%)	556 (87%)	1249 (99%)	1240 (100%)
SPACE
DFT OPT %NPROC=4	-	-	716 (33%)	-	658 (94%)	631 (100%)
SPACE
DFT FREQ %NPROC=1	1479	1373	749	737	2937	3155
SPACE
DFT FREQ %NPROC=2	848 (87%)	-	483 (78%)	432 (85%)	1492 (98%)	1576 (100%)
SPACE
DFT FREQ %NPROC=4	-	-	670 (28%)	-	884 (83%)	889 (89%)

As you can see, it makes sense to run parallely only DFT and HF jobs but not post-HF jobs.