low performance with prepend more than 3 characters in mask attack

[shadowmourne]

when i run

Code:

cudaHashcat64.bin -a 3 -m 5100 b068322473d98c88 123?a?a?a?a?a?a1111111111155e82da34

i got a crack speed of 519 MH/s.

Code:

Session.Name...: cudaHashcat
Status.........: Running
Input.Mode.....: Mask (123?a?a?a?a?a?a1111111111155e82da34) [29]
Hash.Target....: b068322473d98c88
Hash.Type......: Half MD5
Time.Started...: Mon Sep  7 00:16:29 2015 (4 secs)
Time.Estimated.: Mon Sep  7 00:40:07 2015 (23 mins, 32 secs)
Speed.GPU.#1...:   519.0 MH/s
Recovered......: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.......: 2527723520/735091890625 (0.34%)
Rejected.......: 0/2527723520 (0.00%)
Restore.Point..: 26607616/7737809375 (0.34%)
HWMon.GPU.#1...: -1% Util, 61c Temp, N/A Fan

but if i run

Code:

cudaHashcat64.bin -a 3 -m 5100 b068322473d98c88 1234?a?a?a?a?a?a1111111111155e82da3

the crack speed reduced to 30770 kH/s

Code:

Session.Name...: cudaHashcat
Status.........: Aborted
Input.Mode.....: Mask (1234?a?a?a?a?a?a1111111111155e82da3) [29]
Hash.Target....: b068322473d98c88
Hash.Type......: Half MD5
Time.Started...: Mon Sep  7 00:16:12 2015 (7 secs)
Time.Estimated.: Mon Sep  7 07:32:14 2015 (7 hours, 15 mins)
Speed.GPU.#1...: 30770.1 kH/s
Recovered......: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.......: 218365952/735091890625 (0.03%)
Rejected.......: 0/218365952 (0.00%)
Restore.Point..: 218365952/735091890625 (0.03%)
HWMon.GPU.#1...: -1% Util, 54c Temp, N/A Fan

is this a bug or something?