clarified the reason for wait

src/nvim/aucmd.c

@@ -63,10 +63,12 @@ static void do_autocmd_focusgained(bool gained)
                                 NULL, NULL, false, curbuf);
 
   // When activated: Check if any file was modified outside of Vim.
-  // Only do this when not done within the last two seconds (could get
-  // several events in a row).
+  // Only do this when not done within the last two seconds as:
+  // 1. Some filesystems have modification time granularity in seconds. Fat32
+  //    has a granularity of 2 seconds.
+  // 2. We could get multiple notifications in a row.
 
-  if (gained && last_time + (Timestamp)1500 < os_now()) {
+  if (gained && last_time + (Timestamp)2000 < os_now()) {
     need_redraw = check_timestamps(true);
     last_time = os_now();
   }