Will a M1015 in a PCI-E x4 slot run slower and will I see performance degradation during parity build and check with an 8 disk array, including parity disk, attached to this controller? Will an x8 slot perform much faster? All disks are spinners.

It depends 

 

This is a PCIe v2 x8 card ... so it's designed for 8 lanes at 500MB/second/lane.

 

If you connect it to an x4 slot, then as long as the motherboard supports PCIe v2 (or later) the card will have 2GB/s of bandwidth available with 4 lanes.    Clearly that's enough for 8 drives => so the answer is you won't notice any degradation in performance.

 

However, if the motherboard has PCIe v1 slots, the total bandwidth for 4 lanes is only 1GB ... which works out to 125MB/s/drive for 8 drives => this will clearly result in degraded performance when all drives are active at once.

 

FWIW, both of the boards listed in the configs shown in your signature are fine -- the SuperMicro has PCIe v2.0 slots; the AsRock has a PCIe v3.0 slot.

 

I assume you're using the SuperMicro, since the AsRock only has one slot -- and it's an x16 slot    ==> so you should be fine.