I am thinking of finally putting an SSD in my MacPro 5,1

• The SATA II drive bays have a theoretical speed of 300MBps but a SATA SSD would give you about 250MBps real world speed.
• A SATA III controller card has a theoretical speed of 600MBps and a SATA SSD would then give you about 450MBps real world speed.
• An M2 AHCI or NMVe PCIe SSD with a suitable PCIe adapter can give you in theory nearly 1500MBps - much more than that OWCMercury.
• Two M2 PCIe SSDs in a RAID0 stripe array can give you even more than that although this might require using two PCIe slots.

Note: The bigger the capacity of an SSD drive the faster it is, that is a 1TB SSD is faster than a 512GB one which is faster than a 256GB one. My 1TB M2 AHCI SSD gives nearly 1500MBps as above.

The current 'state of the art' solution is to get a PCIe controller for NVMe SSDs cards which supports 'bifurcation'. These PCIe controllers cost a lot more. It would also need to be fitted in slot2 to have access to the extra 'channels'.

See - https://www.amazon.com/CREST-SI-PEX40129-Ports-Bifurcation-Controller/dp/B07HYZY7P2

and - http://www.xlr8yourmac.com/feedback/mac_pro_nvme_pcie_cards/Syba_Dual_M2_NVMe_PCIe_MacPro_Review.html

You may want to check out what I have in my Mac Pro. It is the OWCMercury Accelsior E2. It has nearly twice the speed of any other SSD. For more information read https://www.owcdigital.com/products/mercury-accelsior-e2

Putting a SATA SSD drive is a drive bay is pretty easy. The drives you install on PCIe cards can go faster, but it gets more complicated.

Drives in a PCIE slot that run in the neighborhood of 1,000 M Bytes/sec transfer speeds may need a heatsink to maintain those data rates, else they sometimes slow down to avoid overheating. And there are several different totes and several different connectors to deal with.

The Big advantage to Allan's solution is that it is ready-to-go without any fuss.

I believe 1500MBps would be the max throughput for a standard PCIe SSD controller even for a 2TB drive. However comparing that to SATA II speeds it is clearly a huge improvement and brings the speed to the same level as a 2017 MacBook Pro or iMac. (A 2018 MacBook Pro is faster.)

As I pointed out I have a 1TB SSD which gives that speed and I am happy with it.

The final thing to take in to consideration is that Apple made SSD drives e.g. from a MacBook Pro 2015 or iMac or Mac Pro 2013 use a proprietary Apple version of the M2 connector and hence either need an adapter or a PCIe card designed for that connector. I recommend getting a standard M2 PCIe controller for non Apple NVMe cards.

I also recommend getting a Western Digital Black SN750 NVMe SSD and not a Samsung 970 Evo Pro. The Samsung is known to cause problems in a Mac.

June 2009, I got an 8 core, 2.26 (early 2009 4.1). (8 GB RAM)

April 2012, which I firmware up to 5.1 making it a (mid 2010) 12 core swapped in two CPU 2.93 added (32 GB RAM,)

Oct 2015, upgraded CPU 3.46, (Got a good price on them) But increased speed was disappointing.

Nov 2015, The best thing I did was install a ePCI SSD, (Mercury Accelsior 960/ An old model now). I could not believe the speed that this old machine was able to run at.

Sep 2018, And it is having no problems running Mojave. Of course new Video card metal support (ATI Radeon HD 9750).