On 20.12.2019 12:52, Filipp Gunbin wrote:

>> Filipp, please produce a backtrace during the freeze, either by using
>> SIGUSR2, as Stefan suggested, or by attaching a debugger, and post the
>> backtrace.
> 
> Hi, please find the trace in the attach (it's large).

Thank you. It doesn't exactly answer my question about desktop-read (did 
you really have an old VC-Dir buffer in that saved session?), but it 
shows one bottleneck. One that is even prominently decorated with an old 
comment saying "this is VERY inefficient". :-)

Here's a patch you can try. It should help to an extent. And if it's the 
main bottleneck, we can optimize further, e.g. by eliminating the 
expand-file-name calls.

But if in the end you should see one very long VC-Dir buffer, which can 
be a usability problem on its own.