Poor raidframe reconstruct performance on 8-stable

[Mike Pumford <mpumford%mudcovered.org.uk@localhost>]

Due to a disk failure I'm having to do a full raid1 reconstruct on one 
of my systems and the performance seems much slower than last time I did 
it on this system.

I'm seeing a write speed of 7MB/s which means my 2TB reconstruct is 
going to take 72+hours! Last time I did it it took about 3 hours if I'm 
remembering correctly.

The filesystem level  performance of the raid when its working is a lot 
faster than this this speed.

Build date (Which reflects checkout date is)
NetBSD <hostname> 8.0_STABLE NetBSD 8.0_STABLE (GENERIC) #0: Sat Sep  8 
09:19:05 BST 2018

amd64 system with following cpu:
cpu0 at mainbus0 apid 0
cpu0: Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz, id 0x6fb


Disks are:
wd1 at atabus6 drive 0
wd1: <Hitachi HDS723020BLA642>
wd1: drive supports 16-sector PIO transfers, LBA48 addressing
wd1: 1863 GB, 3876021 cyl, 16 head, 63 sec, 512 bytes/sect x 3907029168 
sectors
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd1(ahcisata1:2:0): using PIO mode 4, DMA mode 2, Ultra-DMA mode 6 
(Ultra/133) (using DMA)
wd2 at atabus7 drive 0
wd2: <Hitachi HDS723020BLA642>
wd2: drive supports 16-sector PIO transfers, LBA48 addressing
wd2: 1863 GB, 3876021 cyl, 16 head, 63 sec, 512 bytes/sect x 3907029168 
sectors
wd2: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd2(ahcisata1:3:0): using PIO mode 4, DMA mode 2, Ultra-DMA mode 6 
(Ultra/133) (using DMA)
raid1: Components: /dev/wd1e /dev/wd2e

Any suggestions?

Mike