Perf comments: getFormatFromFile slows LyX startup considerably

It is currently called on hundreds of files: settings, layouts, icons, cached
graphics files (incl. graphics from files that are not opened on startup).

According to callgrind, fixing the FIXME comments could speed up startup by more
than 30%.

src/ConverterCache.cpp

@@ -159,6 +159,10 @@ void ConverterCache::Impl::readIndex()
 		FormatCache & format_cache = cache[orig_from_name];
 		if (format_cache.from_format.empty())
 			format_cache.from_format =
+				// FIXME perf: This very expensive function is called on all
+				// cached files on opening. This slows LyX startup a lot. It
+				// would be better if this information was retrieved in a
+				// delayed fashion.
 				formats.getFormatFromFile(orig_from_name);
 		format_cache.cache[to_format] = item;
 	}

src/Format.cpp

@@ -531,6 +531,10 @@ bool Formats::isZippedFile(support::FileName const & filename) const {
 	map<string, ZippedInfo>::iterator it = zipped_.find(fname);
 	if (it != zipped_.end() && it->second.timestamp == timestamp)
 		return it->second.zipped;
+	// FIXME perf: This very expensive function is called on startup on each
+	// file whic is going to be parsed, and also on svgz icons. Maybe there is a
+	// quicker way to check whether a file is zipped?  I.e. for icons we
+	// probably just need to check the extension (svgz vs svg).
 	string const & format = getFormatFromFile(filename);
 	bool zipped = (format == "gzip" || format == "zip");
 	zipped_.insert(make_pair(fname, ZippedInfo(zipped, timestamp)));

src/Format.h

@@ -153,6 +153,7 @@ public:
 	 * fails, from file extension.
 	 * \returns file format if it could be found, otherwise an empty
 	 * string.
+	 * This function is expensive.
 	 */
 	std::string getFormatFromFile(support::FileName const & filename) const;
 	/// Finds a format from a file extension. Returns string() if not found.