If i put a Dell H310 in an X4 slot would i loose performance if it had all 8 drives attached? or would loose performance only when all 8 drives are in use? OR would 8 multi TB HDDs even saturate PCIe 2.0 X4?

PCI-e(xpress) Bus (per lane):

v1.x: ~0,250 GByte/s
v2.x: ~0,500 GByte/s
v3.0: ~0,985 GByte/s
v4.0: ~1,969 GByte/s

 

0,5GB x 4 Lanes : 8 drives = 250 MB/s per drive.

This should be enough for a regular spinner drive.

Max speed in a x4 slot with 8 disks is 190MB/s, so enough for most disks.

https://forums.lime-technology.com/topic/41340-satasas-controllers-tested-real-world-max-throughput-during-parity-check/#comment-406521

Would an X4 slot also be capable of supporting a Dual 10 Gbe NIC or would it bottleneck a little bit too?

It can be, since each 10GbE can push 1GB/s and max usable bandwidth of a PCIE 2.0 x4 slot is around 1.6GB/s, but only if would use both ports simultaneously and your hardware is capable of reaching those speeds.

Note- I've tried this. Although the bandwidth should not be a problem that card chokes up very badly on pci2.0x4 lanes on a southbridge.
 

Even with just four drives. Everything slows down. It must need to use all eight lanes regardless of its bandwidth needs, or it’s being throttled by the mobo southbridge that I’ve ran it through. My consumer-grade mobo setup is such that I can’t really test which is happening very easily, and the answer wouldn’t really help me anyway. 

I wanted to contribute what I learned about this because I benefited greatly from this and some other old discussions that I found here. 

Edited July 22, 20205 yr by BrassFox
Clarifications

On 7/19/2020 at 5:10 PM, BrassFox said:

Note- I've tried this. Although the bandwidth should not be a problem that card chokes up very badly on pci2.0x4 lanes on a southbridge.
 

Even with just four drives. Everything slows down. It must need to use all eight lanes regardless of its bandwidth needs, or it’s being throttled by the mobo southbridge that I’ve ran it through. My consumer-grade mobo setup is such that I can’t really test which is happening very easily, and the answer wouldn’t really help me anyway. 

I wanted to contribute what I learned about this because I benefited greatly from this and some other old discussions that I found here. 

I think the bottleneck is related to the southbridge, because in my system it does not makes difference in x8 vs x4 during parity checks or turbo writes.  (max 190mb/s with all drives).  LSI SAS9211-8I is connected to a cpu pcie.