diff --git a/languages/clang/duchain/clangindex.cpp b/languages/clang/duchain/clangindex.cpp
index 02502191dd..e8e4ba21b7 100644
--- a/languages/clang/duchain/clangindex.cpp
+++ b/languages/clang/duchain/clangindex.cpp
@@ -1,114 +1,119 @@
 /*
  *    This file is part of KDevelop
  *
  *    Copyright 2013 Olivier de Gaalon <olivier.jg@gmail.com>
  *
  *    This library is free software; you can redistribute it and/or
  *    modify it under the terms of the GNU Library General Public
  *    License as published by the Free Software Foundation; either
  *    version 2 of the License, or (at your option) any later version.
  *
  *    This library is distributed in the hope that it will be useful,
  *    but WITHOUT ANY WARRANTY; without even the implied warranty of
  *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  *    Library General Public License for more details.
  *
  *    You should have received a copy of the GNU Library General Public License
  *    along with this library; see the file COPYING.LIB.  If not, write to
  *    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  *    Boston, MA 02110-1301, USA.
  */
 
 #include "clangindex.h"
 
 #include "clangpch.h"
 #include "clangparsingenvironment.h"
 #include "documentfinderhelpers.h"
 
 #include <util/path.h>
 #include <util/clangtypes.h>
 #include <util/clangdebug.h>
 #include <language/backgroundparser/urlparselock.h>
 
 #include <clang-c/Index.h>
 
 using namespace KDevelop;
 
 ClangIndex::ClangIndex()
     // NOTE: We don't exclude PCH declarations. That way we could retrieve imports manually, as clang_getInclusions returns nothing on reparse with CXTranslationUnit_PrecompiledPreamble flag.
     : m_index(clang_createIndex(0 /*Exclude PCH Decls*/, qEnvironmentVariableIsSet("KDEV_CLANG_DISPLAY_DIAGS") /*Display diags*/))
 {
+    // demote the priority of the clang parse threads to reduce potential UI lockups
+    // but the code completion threads still retain their normal priority to return
+    // the results as quickly as possible
+    clang_CXIndex_setGlobalOptions(m_index, clang_CXIndex_getGlobalOptions(m_index)
+        | CXGlobalOpt_ThreadBackgroundPriorityForIndexing);
 }
 
 CXIndex ClangIndex::index() const
 {
     return m_index;
 }
 
 QSharedPointer<const ClangPCH> ClangIndex::pch(const ClangParsingEnvironment& environment)
 {
     const auto& pchInclude = environment.pchInclude();
     if (!pchInclude.isValid()) {
         return {};
     }
 
     UrlParseLock pchLock(IndexedString(pchInclude.pathOrUrl()));
 
     static const QString pchExt = QStringLiteral(".pch");
 
     if (QFile::exists(pchInclude.toLocalFile() + pchExt)) {
         QReadLocker lock(&m_pchLock);
         auto pch = m_pch.constFind(pchInclude);
         if (pch != m_pch.constEnd()) {
             return pch.value();
         }
     }
 
     auto pch = QSharedPointer<ClangPCH>::create(environment, this);
     QWriteLocker lock(&m_pchLock);
     m_pch.insert(pchInclude, pch);
     return pch;
 }
 
 ClangIndex::~ClangIndex()
 {
     clang_disposeIndex(m_index);
 }
 
 IndexedString ClangIndex::translationUnitForUrl(const IndexedString& url)
 {
     { // try explicit pin data first
         QMutexLocker lock(&m_mappingMutex);
         auto tu = m_tuForUrl.find(url);
         if (tu != m_tuForUrl.end()) {
             if (!QFile::exists(tu.value().str())) {
                 // TU doesn't exist, unpin
                 m_tuForUrl.erase(tu);
                 return url;
             }
             return tu.value();
         }
     }
     // otherwise, fallback to a simple buddy search for headers
     if (ClangHelpers::isHeader(url.str())) {
         foreach(const QUrl& buddy, DocumentFinderHelpers::getPotentialBuddies(url.toUrl(), false)) {
             const QString buddyPath = buddy.toLocalFile();
             if (QFile::exists(buddyPath)) {
                 return IndexedString(buddyPath);
             }
         }
     }
     return url;
 }
 
 void ClangIndex::pinTranslationUnitForUrl(const IndexedString& tu, const IndexedString& url)
 {
     QMutexLocker lock(&m_mappingMutex);
     m_tuForUrl.insert(url, tu);
 }
 
 void ClangIndex::unpinTranslationUnitForUrl(const IndexedString& url)
 {
     QMutexLocker lock(&m_mappingMutex);
     m_tuForUrl.remove(url);
 }