low speed. brute force with a mask

[stafford310i]

When testing md5, my speed is 1600 mhash / s, but when using brute force, the speed drops dramatically, 8.7 mhash / s. I don’t understand what this is connected with, I use 5 static characters and a mask, for example 12345?d?d?d?d? d?d?d?d, at the same time low speed. Testing found that in version 5.1.0 if there is one static character, then the speed does not drop, if two drops to 500 mhash / s, if there are more unchanged characters, the speed is always 8.7 mhash / s. But in version 3.6, the speed does not drop with two characters, with three 500 mhash / s, more than 8.7 mhash / s fixed characters.
example: v5.1.0
-a 3 -m 0 -O -w 3 hash 12345?d?d?d?d?d?d?d?d
speed 8.7 mhash / s
-a 3 -m 0 -O -w 3 hash 123?d?d?d?d?d?d?d?d
speed 8.7 mhash / s
-a 3 -m 0 -O -w 3 hash 12?d?d?d?d?d?d?d?d
speed 500 mhash / s
-a 3 -m 0 -O -w 3 hash 1?d?d?d?d?d?d?d?d
speed 1600 mhash / s

v3.6.0
-a 3 -m 0 -O -w 3 hash 12345?d?d?d?d?d?d?d?d
speed 8.7 mhash / s
-a 3 -m 0 -O -w 3 hash 1234?d?d?d?d?d?d?d?d
speed 8.7 mhash / s
-a 3 -m 0 -O -w 3 hash 123?d?d?d?d?d?d?d?d
speed 500 mhash / s
-a 3 -m 0 -O -w 3 hash 12?d?d?d?d?d?d?d?d
speed 1600 mhash / s
-a 3 -m 0 -O -w 3 hash 1?d?d?d?d?d?d?d?d
speed 1600 mhash / s


How can I get full speed and is it possible?
Why does it work differently in different versions?
I hope for help.
Thank you in advance for any help.
Sorry for the text, I am writing through a Google translator.

[stafford310i]

windows 7 64, hashcat 5.1.0 gpu gtx770
when the mask is
1234?d?d?d?d?d?d the work speed is very slow
when the mask ?d?d?d?d?d?d1234 speed fast