You can carry a small grid past 3km/s, however the small grid's speed will immediately decelerate to about 3km/s (you can still have small grids going over 3km/s, they just won't accelerate).
Having multiple small grids over 3km/s or having a small and large grid crash at over 3km/s will seriously lag the game though.

SE loading quote:
"One of the greatest pleasures in life is doing what others say you can not do."
Midspace:
"You will not be able to keep a small ship docked to a large ship over 3000 m/s."
Me:
"We'll see about that..."

Thanks mate! :)

Adding mods to your game
1) Go to Steam Workshop, select mods (right part of screen), subscribe to mods you want
2) Start Space Engineers
3) Select New Game or Load game
4) Go to Edit Settings
5) Click "Mods" button
6) Double-click mods you want to have active for current world
7) Start the game!

Taken from the official guide on how to add mods to your game.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=296249959

How Do you work this?

@COMMANDER Striker : I have a configurable speed mod now, so *you* can choose the speeds you want.
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=599536562

the speed is nice but you should swich the speeds large slower small faster

this sounds like a huge mess

I love this, its perfect!

I've seen what happens when a ship docked inside a larger ship freaks out....everything goes wrong.....everything turns into a destructive nightmare. Then you watch all your hard work get obliterated into pieces...

@ulfsark9 : the small ship will break free and continue travel in the same direction at that limited speed. Just make sure it isn't inside or in front.

I love you long time.

Now what happens when a small ship is docked to a large one, and the large one moves past 3km/s ? :P

Wahahahahahahahaha!!! This is too good!!! Ahahahaha!!!