so quick update here. I plugged everything into a spare laptop I had and everything works fine - the fact that the program crashes every once and a while. then I reinstalled windows on my desktop and its still lagging so my theory is its windows 10 that's the issue.

You have several issues working against you, so let's go through them one at a time.
First, when the mouse settings feel different while connected to the PC, it means your Windows mouse settings are influencing your mouse input into the KeyMander. You need to change your Windows mouse settings to 6/11 and turn off "enhance pointer precision" as described here.

Next, the mouse isn't laggy, what you are experiencing is having the mouse sensitivity, deadzone and/or acceleration set too high and exceeding the game's maximum turn speed as described here. Once you have exceeded the maximum input speed for the game, the faster you move the mouse, the slower it will feel in game. You need to start by using the sample profile for your game and using the in-game settings as listed on the sample profile. Once you have a feel for things you can start making adjustments, but you must learn to work within the game's maximum turn speed as explained in the link in this paragraph.

The G600 has on-board memory, but it must be set up in the Logitech software as "persistent" profile so that whatever macros or button bindings you make will stay saved in the mouse. Normally I suggest using the KeyMander software to do this, and if you are programming the mouse side buttons you will select the "Forward" and "Back" buttons in the drop down menu.

A laggy laptop will cause issues with keystroke registration, especially for movement with the WASD keys, but usually does not affect mouse settings because of the slow processing. The mouse issues are normally caused by driver conflicts which is normally the result of Windows mouse settings. Check those as described above and let us know.

is there a way to use the side panel without pc if not im returning the item, ive done everything you mentioned above and it doesn't work

also side note im using my high end desktop for this so theres no lag issues

Yes, you can certainly use the mouse side panel buttons without having the PC attached, but you do need to use it or an Android device to set the button functions up and save the profile to the KeyMander.
Do you already have your profile set up the way you want except for the mouse side buttons? If so, what commands do you want on the side buttons and I'll give you the steps to make them work. In the meantime I will look for our G600 mouse to verify the two different methods we normally use for this.

Could I get the steps as well please?

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