I just switch a 600gig drives to a 4TB drive and nothing is in the file and it still going to take 6 days to rebuild. 

 

Total size:4 TB

Elapsed time:1 hour, 25 minutes

Current position:33.0 GB (0.8 %)

Estimated speed:8.3 MB/sec

Estimated finish: 5 days

zeroflix-diagnostics-20190825-1503.zip

You have a lots of memory errors like this:

OVERFLOW area:DRAM err_code:0001:0093 socket:0 ha:0 channel_mask:8 rank:1)
Aug 25 06:06:35 ZeroFlix kernel: mpt2sas_cm0: host_add: handle(0x0001), sas_addr(0x500605b006501f60), phys(8)
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: HANDLING MCE MEMORY ERROR
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: CPU 17: Machine Check Event: 0 Bank 5: cc10128000010093
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: TSC 0 
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: ADDR 8584292c0 
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: MISC 2440149486 
Aug 25 06:06:35 ZeroFlix kernel: EDAC sbridge MC0: PROCESSOR 0:206d7 TIME 1566738366 SOCKET 0 APIC 3
Aug 25 06:06:35 ZeroFlix kernel: EDAC MC0: 16458 CE memory read error on CPU_SrcID#0_Ha#0_Chan#3_DIMM#0 (channel:3 slot:0 page:0x858429 offset:0x2c0 grain:32 syndrome:0x0 -  

I would address this issue and see if the rebuild speed problem is resolved. 

I would look into that Machine Check Event. I was getting those in my log file a while back and ended up having to buy a new CPU. Bad news, I know, but on the bright side, Shiny new toy!!

 

OTOH, it could be a memory issue.