Mojave glacially slow, will not access internet
I am having persistent problems with upgrade from High Sierra to Mojave. My computer is a Mac Pro Tower 5,1 (2012) with dual 6-core 3.46 GHz Intel Xeon 5690 processors, 32 GB 1333 MHz DDR3 RAM, a 1 TB Samsung EVO 840 SSD with 640 GB of free space, 2x 1TB internal HDs, several external USB drives, and an MSI graphics card running an AMD Radeon HD 7950 chipset which is metal capable. The card is flashed for EFI (not UEFI), and does provide a boot screen when pressing “option” on start. FileVault is off (I have never used it). The boot ROM is 138.0.0.0.0, and the SMC is 1.39f11. I am trying to use Mojave with a Samsung EVO 840 1 TB SSD. However, I get identical results with any HD I have tried as well.
The system works perfectly with High Sierra installed. However, when booting from Mojave it is glacial. Once my log-in password is entered (entry of characters is very slow) and the system finally boots to the Finder, operations are extremely slow and fundamentally unusable. Software such as Microsoft Word and Apple programs open after very long delays. Most importantly, though my ethernet and Wi-Fi-connected internet access functions perfectly in High Sierra, Mojave shows no internet available at all, including any data whatsoever in the System Preferences > Network pane. When open, the network pane will accept NO input. No button functions in that pane.
I have spent two weeks, including six days in chat and on the phone with Apple support, and took the computer to an authorized Apple repair shop, with no result. I went so far as to transfer the metal-capable MSI Radeon HD 7950 card in a Mac Pro 4,1 tower that had been flashed to a 5,1 configuration, and that also has boot rom 138.0.0.0.0 loaded. That computer has a 12 core 3.33 GHz Intel Xeon 5680 chipset and 16 GB of 1333 MHz DDR3 RAM. Although that computer also runs perfectly on both Sierra and High Sierra, booting from the same EVO 840 SSD with Mojave installed yielded exactly the same result as on the true 5,1 2012 tower. I would suspect the SSD, but I get the same result with an HD.
To try to fix the glacial response I have: Reset the SMC; reset NVRAM; used both Disc Utility and Recovery mode to try to fix the SSD (it registers OK); tried SAFE mode; and reinstalled Mojave after booting into Recovery mode; trashing the Mojave installer download and starting again from the App store, and installing it clean, bringing my files over later. Nothing has worked. Any suggestions?
Mac Pro, macOS High Sierra (10.13), Dual six core 3.33 GHz, 32 GB RAM