Dear Thierry,
Something is wrong with at least one of these timings. The real time can never be shorter than the CPU time if you are using only 1 core. Yet in your table there is an entry where the real time increment is only 5 seconds, while the CPU time is
2724 s.
You should be able to confirm that it is really only 1 thread running with something like htop (if that's available on Mac).
With that said, I recommend checking your disk utilization while Molpro is running (in terms of disk busy%).
The most likely reason for real time being much higher than CPU time (especially when running on only 1 core) is a disk bandwidth bottleneck.
This can be exacerbated by giving Molpro either too much or too little memory, but some methods in Molpro always do a lot of disk IO.
If you are indeed IO-bound, I have no good recommendations I am afraid.
Under Linux you can work around this by setting some kernel tunables to maximize write caching by the OS in free RAM. But this might not be possible under MacOS.
And in general I would caution against running IO heavy quantum chemistry on Macs, unless you are mindful of the impact it can have on the lifespan of your SSD. Like most SSDs, the Mac's storage is made out of NAND chips, which have a rather finite write endurance. Every write to disk damages the NAND chips a tiny amount, which is usually not a concern, but workloads with a lot of disk writes WILL wear out your storage chips. This is especially painful on modern Macs, where replacing the storage chips is unreasonably costly, even in cases where they are on an easy-to-replace module.
The situation is even worse for Macbooks and other Macs where the NAND chips are soldered to the logic board.
I guess one option would be to buy a high-quality (cooling!) Thunderbolt 40 Gbps M.2 NVMe SSD enclosure, fit that with a fast NVMe SSD, and use that external drive as your Molpro scratch drive. This would also spare the expensive internal SSD from being worn down by the large amount of writes.
If that is still not sufficient bandwidth, you could try a Thunderbolt 5 80 Gbps external enclosure (if your Mac has TB5) or multiple TB4 SSDs in RAID 0.
Best,
Tibor