How can I reduce the ever-increasing time to push a subtree?

I'm using Git subtree to "link" several subprojects into the main project (I'm coming from "svn:externals"). I've used it for some weeks, but the time to push changes to the subtree remote increases every commit.

$ git subtree push -P platform/rtos rtos master

git push using:  rtos master

1/    215 (0)2/    215 (1)3/    215 (2)4/    215 (3)5/    215 (4)6/    215 (5)7/    215 (6)8/    215 (7)9/    215 (8)10/    215 (9)11/    215 (9)12/    215 (10)13/    215 (11)14/    
...
20 more lines
...
(204)209/    215 (205)210/    215 (206)211/    215 (207)212/    215 (208)213/    215 (209)214/    215 (210)215/    215 (211)To https://github.com/rtos/rtos.git
   64546f..9454ce  9a9d34c5656655656565676768887899898767667348590 -> master

Is there any way to "clean up" the subtree and therefore reduce the time to push changes?

Solution

Note that if you decide to switch to git submodule, you now can, since git1.8.2 (2013-03-08) track the latest commits of a submodule repo.

See git externals.

"git submodule" started learning a new mode to integrate with the tip of the remote branch (as opposed to integrating with the commit recorded in the superproject's gitlink).

That could be make for quicker push, while benefiting from the additional information a submodule has over a subtree (i.e a lightweight record of a specific commit of the submodule)

You can update that submodule to the latest of a given branch with:

git submodule update --remote

This option is only valid for the update command.
Instead of using the superproject's recorded SHA-1 to update the submodule, use the status of the submodule's remote tracking branch.