[Qgis-developer] Poor performance with large projects

[Sam Ingarfield]

Hi Everyone,

I have been suffering from *very* poor performance on a number of projects,
which only seems to become noticeable on large, complex projects. I have
filed a bug with regards to this, see http://hub.qgis.org/issues/12125.
Giovanni suggested I cross-post on the developer and user lists to see if
others are experiencing the problem/have any ideas on what's going on.

The issue I am experiencing manifests in two ways:
a) Very slow project loading times (>30mins at times). Someone on the bug
tracker suggested that this may in part be related to the retrieval of
extent and geometry from postgis layers. I've had a look at the debugging
logs (from the master build) and this seems to be the case, for what I
estimate as about half the loading time.
b) Very slow addition and removal of layers after the first save event.
>From a quick look at the debug output, it seems that after the first save,
the entire project is being reloaded on each layer addition/removal, which
takes a very long time. Before the first save event, performance is very
good, addition and removal of layers is virtually instantaneous.

Obviously long project times are annoying but manageable, so long as I only
have to load the project infrequently. The second issue, however, makes
Qgis pretty much unworkable and leads to very long times between saves (and
the inevitable quit & reload to restore performance) which is poor practice.

To give you an idea of the difference between pre and post-save behaviour,
Qgis emits about 850 lines of debugging output pre-save, rising to over
430,000 lines post-save when adding a layer to one of my projects.

Am happy to share debugging output with a developer off-list if that would
help nail down what is happening and could lead to a fix.

Cheers;
Sam Ingarfield.
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