Trying to increase performance of mask attack with a two character prefix

Hi,

I am completely new to hashcat and am trying process as many MD5 hashes of the input "0.################" per second, where '#' is a digit 0-9. The mask I am currently using to attempt this is "0.?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d", however this is significantly slower than if I use the mask "?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d" which does not include the prefix. I am aware that the first four characters in the mask should allow for lots of possible combinations to increase GPU utilisation. I have seen that using the ($salt.$pass) variation of the MD5 algorithm can be a possible solution however this has not been the case for me.

Using the non-prefix mask I achieve ~650 MH/s
Using the prefix mask I achieve ~420 MH/s
Using the salt method (-m 20) I achieve ~280 MH/s

Would love to know how I can fix this if possible

Thanks