fix #1165: insert mode lag

autoload/UltiSnips.vim

@@ -3,6 +3,9 @@ if exists("b:did_autoload_ultisnips")
 endif
 let b:did_autoload_ultisnips = 1
 
+" Ensure snippets are loaded for current buffer
+au UltiSnips_AutoTrigger FileType,BufEnter * call UltiSnips#CheckFiletype()
+
 " Also import vim as we expect it to be imported in many places.
 py3 import vim
 py3 from UltiSnips import UltiSnips_Manager
@@ -183,6 +186,10 @@ function! UltiSnips#TrackChange() abort
     py3 UltiSnips_Manager._track_change()
 endfunction
 
+function! UltiSnips#CheckFiletype() abort
+    py3 UltiSnips_Manager._check_filetype(vim.eval('&ft'))
+endfunction
+
 function! UltiSnips#RefreshSnippets() abort
     py3 UltiSnips_Manager._refresh_snippets()
 endfunction

pythonx/UltiSnips/snippet/source/base.py

@@ -15,6 +15,7 @@ class SnippetSource:
     def __init__(self):
         self._snippets = defaultdict(SnippetDictionary)
         self._extends = defaultdict(set)
+        self.must_ensure = True
 
     def ensure(self, filetypes):
         """Ensures that snippets are loaded."""

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

@@ -27,9 +27,11 @@ def __init__(self):
         SnippetSource.__init__(self)
 
     def ensure(self, filetypes):
-        for ft in self.get_deep_extends(filetypes):
-            if self._needs_update(ft):
-                self._load_snippets_for(ft)
+        if self.must_ensure:
+            for ft in self.get_deep_extends(filetypes):
+                if self._needs_update(ft):
+                    self._load_snippets_for(ft)
+            self.must_ensure = False
 
     def refresh(self):
         self.__init__()

pythonx/UltiSnips/snippet_manager.py

@@ -130,6 +130,7 @@ def __init__(self, expand_trigger, forward_trigger, backward_trigger):
         self._visual_content = VisualContentPreserver()
 
         self._snippet_sources = []
+        self._filetypes = []
 
         self._snip_expanded_in_action = False
         self._inside_action = False
@@ -975,6 +976,15 @@ def _refresh_snippets(self):
             source.refresh()
 
 
+    @err_to_scratch_buffer.wrap
+    def _check_filetype(self, ft):
+        """Ensure snippets are loaded for the current filetype."""
+        if ft not in self._filetypes:
+            self._filetypes.append(ft)
+            for _, source in self._snippet_sources:
+                source.must_ensure = True
+
+
 UltiSnips_Manager = SnippetManager(  # pylint:disable=invalid-name
     vim.eval("g:UltiSnipsExpandTrigger"),
     vim.eval("g:UltiSnipsJumpForwardTrigger"),