All the controls seem OK apart from walk which seems to be at a crawl space. I've had this game running a number of times over the years so I'm sure it's not "me" :)

Check your user bindings. They got changed recently since the release of NewDark, which means they will also be different in the GOG release.

Upgrade agility?

Keep in mind that there's both a run and a walk function, so if you're using Walk it will feel pretty slow.

I'm plagued with the same issue, I walk at a snail pace. Moving backwards and strafing are alright and if I press shift I go even SLOWER if that is ever possible.

Bind W to "Run Forward," not "Walk Forward." Run by default. Hold shift to walk.

And, an fov of 110 to 120 will make it feel a lot faster also.

I change the settings so that up is run but now I move forward faster than left / right / backwards and it feels a bit odd. Plus I kind of like moving through the game walking where possible as it intensifies the tense atmosphere!

Bind speed modifier to Shift instead of walk to Shift+walk. When you hold down Shift, as the speed modifier, it is a bit faster than walk, it is more the speed you would expect.