Problem: it’s not possible to seek file position during 2X speed playback.
Related function: gst_soup_http_src_do_seek(…)

Actually, this problem is caused by below routine,
if (segment->rate != 1.0 || segment->format != GST_FORMAT_BYTES) {
+    GST_WARNING_OBJECT (src, "Invalid seek segment");
     return FALSE;
+  }
As you can see, the do_seek is allowed only when “rate” is 1.0.

I’ve reviewed both commit log and bug report but I don’t know why you prohibit the seek condition in double speed playback.
Commit log: http://cgit.freedesktop.org/gstreamer/gst-plugins-good/commit/ext/soup?id=6a9372529222300c7b6eacfcf8b9a844b96021c9
Related bug: https://bugzilla.gnome.org/show_bug.cgi?id=632977

So I’d like to know, what happened will be occurred, if I removed the condition regarding playback speed.

BR,
Davy