Does the issue resolve itself after a time increment? (Theoretically) as long as you've got "use max speed" checked, the fleet should speed back up to max speed once they get underway.

I've seen similar display issues when giving orders to tugs that had dropped off terraformers - their orders will give an ETA that's based on their laden speed, but they'll actually get there much faster.

Negative.
The issue persists after time is incremented.
Move orders given to such a fleet will be undertaken at 1km/s.

Merging fleets with tugs can be problematic.

Say Fleet A has one or more ships with a tractor beam, and all these ships are tractoring engineless ships.

If Fleet A executes an order to join some Fleet B (which may or may not also have ships with engaged tractor beams), then sometimes (not always) the speed of Fleet B drops to 1km/s.

If I drag Fleet A into Fleet B, same thing happens: sometimes (not always) the speed of Fleet B drops to 1km/s.

The only way to reliably join the fleets without reducing the speed of Fleet B to 1 km/s is to individually drag the tractors from Fleet A to Fleet B in the left hand tree view.
(Dragging this way automatically brings the tractored ships along.)

Obviously, this can be very tedious for large fleets, and can only be done when the fleets are in the same location.
And it means that you just can't use the Join Fleet order when tugging ships.
And I do a *lot* of tugging, so this adds a lot of micromanaging to my playtime.

I'm hoping there's something easy to tweak in the fleet merging logic that can resolve this.
I wonder if the resulting speed is being determined by iterating through the list of all ships, which might be in ascending order of ship construction time, and it doesn't recognize that a ship is tractored unless the tractor has already been processed.

Note: I don't know if the situation only occurs when engineless tractors are targeted, because I only very rarely tractor ships with engines.

Other note: the same thing happens when detaching subfleets with tractors from fleets. Sometimes the newly detached fleet has a speed of 1km/s. Perhaps the same code is handling this as well.

While playing through a combat, I noticed that an enemy ship was not closing on my own at the expected rate.

I investigated, gathered the details, and began typing up a bug report:

Distance traveled over time does not reconcile with movement speed.

Situation:
My missile escort and an enemy contact are moving directly towards each other.
My escort speed is 1493 km/s.
Enemy contact speed is 5377 km/s.
Net expected closing speed is therefore 1493+5377 = 6870 km/s.
Net expected range reduction per 5s increment is 6870 * 5 = 34350km.

At 09:55:15, I launched AMM missiles at the contact. The Events window reported the range as 6,051,747km.
At 09:55:20, I launched ASM missiles at the contact. The Events window reported the range as 6,032,327km.

Those range reports indicate a net range reduction of 19420km in 5s.
This does not match the expected range reduction of 34350km.
(The reported speed of the enemy contact has not changed, either just prior to firing or during the single intervening increment.)

The reported range reduction implies a net closing speed of 19420/5 = 3884km/s (about 55% of expected).

Fast forward to missile impact.
My AMMs have a speed of 14600km/s.
My ASMs have a speed of 14595km/s.

At 10:00:20, the AMMs arrive on target.
That's an elapsed time of 305 seconds, for a calculated distance of 14600 km/s * 305s = 4453000km = 4.453mkm

At 10:00:25, the ASMs arrive on target.
That's an elapsed time of 305 seconds, for a calculated distance of 14595 km/s * 305s = 4451475km = 4.451mkm

At the time of ASM arrival, my escort's fire controls showed a range of 4.85mkm.
This range reduction over 305 seconds also implies a net closing speed of 3884km/s.
It seems as if

And that's when I realized what was going on.
My missile escort move command was not "Move To Location", it was "Follow" (at 6.5mkm distance).

The actual net closing speed was not the sum of the two fleet speeds, it was the enemy's speed minus my escort's speed.
5377-1493 = 3884.

In other words, this turned out not to be a weird bug, but a straightforward PICNIC.

Oh I see, in the RIGHT hand list! It never occurred to me to test multi-detach from there, thanks!

Yes, the tree view is a Microsoft control and it doesn't allow multi-select.

In the Environment tab, together with the Annual Terraform capacity, I would like to read the number of the Terraforming Istallations active on (or near) a body.

Added for v2.6.