I noticed this long time ago already and it's quite annoying and needs to be fixed. My first guess without having done any investigations, we are doing too much UI updating stuff on bulk loading of files (e.g. the previously mentioned resorting of the document list, there are probably many more such issues).
I think it isn't only UI stuff because my system monitor shows me (lot of) I/O, that seems to be heavy disk *writes*, in addition to simple CPU usage. Note that this happens effectively only after the files being loaded (hum, after the info about them are shown).
Haha, got it! Thanks to gdb, I found what takes all this time: GtkRecentManager, and particularly gtk_recent_manager_add_item(manager, uri) at line 1037 of ui_utils.c. Why? well, it seems to syncs its state at each and every addition, and the g_file_set_contents(), which calls… fsync(). Commenting the addition completely fixes the problem after file loading. Now it takes about 11 seconds, to load the files (always as reported by the verbose messages). Time to search for a but report on GTK I think.