It's hard to wait, so I found my way to solve this problem.

Any ideas how to fix it?

In T5896#7, @epriestley wrote:

Do you have ~10,000 branch heads or bookmarks in this repository?

Trace: {F192790}
I tried Phab with CentOS7 and python 2.7.5 and hg 2.6.2 on the same repo, and have the same result
In branch history there is offset 1100, so it's near this number of branch heads

http://phabricator/diffusion/F/branches/default/?offset=1100

But

[root@Phabricator F]# hg branches | wc -l
221

And there are no bookmarks