do not walk &rtp if abs path to snips specified

doc/UltiSnips.txt

@@ -193,6 +193,13 @@ g:UltiSnipsSnippetsDir
                             directories named "snippets" are reserved for
                             snipMate snippets and cannot be used.
 
+                                                        *g:UltiSnipsSnippetDirectories*
+g:UltiSnipsSnippetDirectories
+                            Defines the directories for looking for snippets.
+                            Do not mix up this variable with previous one.
+                            More information about that variable can
+                            be found at section |UltiSnips-snippet-search-path|.
+
                                                         *g:UltiSnipsEnableSnipMate*
 g:UltiSnipsEnableSnipMate
                             Enable looking for SnipMate snippets in
@@ -350,6 +357,15 @@ over the global variable.
 
 |UltiSnips-adding-snippets| explains which files are parsed for a given filetype.
 
+If only one directory is specified in this variable and this directory is
+specified by absolute path, UltiSnips will not look for snippets in
+&runtimepath, which can lead to significant speedup. So, the common case is:
+
+    let g:UltiSnipsSnippetDirectories=[$HOME.'/.vim/UltiSnips']
+
+However, you will not able to use snippets that are shipped with third party
+plugins out of the box. You'll need to copy them into your chosen directory.
+
 
 3.4 Warning About Select Mode Mappings          *UltiSnips-warning-smappings*
 --------------------------------------

pythonx/UltiSnips/snippet/source/file/ultisnips.py

@@ -32,10 +32,13 @@ def find_all_snippet_files(ft):
         snippet_dirs = _vim.eval('b:UltiSnipsSnippetDirectories')
     else:
         snippet_dirs = _vim.eval('g:UltiSnipsSnippetDirectories')
-
+    if len(snippet_dirs) == 1 and os.path.isabs(snippet_dirs[0]):
+        check_dirs = ['']
+    else:
+        check_dirs = _vim.eval('&runtimepath').split(',')
     patterns = ['%s.snippets', '%s_*.snippets', os.path.join('%s', '*')]
     ret = set()
-    for rtp in _vim.eval('&runtimepath').split(','):
+    for rtp in check_dirs:
         for snippet_dir in snippet_dirs:
             if snippet_dir == 'snippets':
                 raise RuntimeError(