GoXDCR: Frequent "Failed to resend document" errors result in very slow replication

Build

4.0.0-2036

-found during manual testing

-simple uni-xdcr with checkpoint_interval =60s
-loaded 37840 keys on src. All keys got replicated
-flushed target bucket
-after a min, all keys got replicated
-flushed target bucket again
-replication very slow and is seen in bursts (37k keys take more than 5-6 mins), see pipeline getting constructed with errors like

2015-05-04 17:25:00 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_1:Failed to resend document aruna3.9482, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:23:41 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_1:Failed to resend document aruna3.18083, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:22:22 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.188:11210_1:Failed to resend document aruna3.6843, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:21:03 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_1:Failed to resend document aruna3.7037, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:19:43 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.188:11210_1:Failed to resend document aruna3.6191, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:18:24 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.188:11210_1:Failed to resend document aruna3.33717, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:17:04 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_1:Failed to resend document aruna3.12977, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:16:49 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_1:Failed to resend document aruna3.25659, has tried to resend it 6, maximum retry 5 reached]
2015-05-04 17:15:30 map[xmem_672645dcb5e47d01f46a75ac0c187d2c/default/default_10.3.4.189:11210_0:Failed to resend document aruna3.37792, has tried to resend it 6, maximum retry 5 reached]

Why do we see this error? I see this in many scenarios. There was no load or topology change on target cluster except for the flush.