For anyone else looking for the answer.
I searched google looking to enable the gamespeed slider in yuri's revenge but probably like yourself, didn't find a whole lot.
I'm currently procrastinating from my studies by playing RA2 + Yuris revenge(procrastination is somewhat of a talent of mine....) ......luckily my study topic is reverse engineering.
If you have the yuri's revenge game version 1.001 (EA Origin), then all you need to do to enable the gamespeed slider is replace the two bytes '74 07' with '90 90' at the offset 00115439 in a hex editor for the file gamemd.exe .....just like with RA2..(naturally the code is in a different place as its a different game).
74 is the JMP instruction, 90 is the NOP (no operation) instruction, all this does is blank out the part of the code that makes the game evaluate if the gamespeed command line parameter was passed, and sets it to 1.
Hope that helps