The current rules penalise terminator armour on speed, fair enough. But I think they take it too far. The agility bonus penalty or the lack of running would do well enough by themselves.

So a proposal:

A brother equipped with Terminator Armour may run but *only* in a straight line. And with the AB penalty.

E.g. Brother Bethor normally has Ag 40, so a AB of 4. In Power Armour, this increases to AB 5 for movement, so a full move would be 10m and a run 30m.

In Terminator Armour, this drops to AB 2. So a full move would be a grand total of...4m. And no run.

This house rule would allow a run for a total of 12m. Which is still barely above the full move in power armour. A significant movement penalty but one that means terminators will get to their destination in a sane amount of time.

I'm sorry, I just can't picture how a "running" Terminator would look like when he attempts to cover the same distance as a walking Marine just by moving his legs very fast.

I know some people really dislike the mechanical disadvantage of slow-moving joints, but ...

... well, if you feel you need it, that's what house rules are for. Your suggested approach is certainly not a game breaker or in any way imbalanced. I'm just convinced it would be a bad representation of how such a suit of armour is moving, as per its original description. But since background details are a matter of interpretation as well, perhaps you could change the description to suit your preferences?

Personally, if I really wanted to make the suit faster, I'd simply reduce the AB penalty, but keep the "no run". If your concern is based on covering distances, perhaps this would be a way to maintain the suit's original description whilst still making it faster?

My thinking behind the "running" house rule is that I can easily imagine Terminators building up momentum. Ducking and weaving is out, but sheer forward charging does work to my mind. But fair point, more food for thought.

Yeah, it's primarily just a matter of how we imagine them moving. I confess to having a specific image in my mind ever after seeing them in that old live action ad GW did ages ago. Much of that may have been due to their budget for these short movies, but it just happens to coincide 100% with their codex description, too.

https://www.youtube.com/watch?v=gZBplpT3Iy4

So in my mind, Terminators are to 40k what the Power Lifter is to the Aliens franchise - just with way more armour and better weapons.