Last version tested: 11.13
The source release appears not to actually include the logic that performs operations in parallel, so this patch doesn't actually do anything.
diff -Naur -x '*.orig' -x '*.rej' -x '*.git*' 11.4-0/FreeFileSync/Source/ui/folder_selector.cpp 11.4-1/FreeFileSync/Source/ui/folder_selector.cpp
--- 11.4-0/FreeFileSync/Source/ui/folder_selector.cpp	2020-12-08 08:15:29.428156465 -0500
+++ 11.4-1/FreeFileSync/Source/ui/folder_selector.cpp	2020-12-08 19:12:09.205183330 -0500
@@ -256,7 +256,7 @@
 void FolderSelector::onSelectAltFolder(wxCommandEvent& event)
 {
     Zstring folderPathPhrase = getPath();
-    size_t parallelOps = getDeviceParallelOps_ ? getDeviceParallelOps_(folderPathPhrase) : 1;
+    size_t parallelOps = getDeviceParallelOps_ ? getDeviceParallelOps_(folderPathPhrase) : 5;
 
     if (showCloudSetupDialog(parent_, folderPathPhrase, sftpKeyFileLastSelected_, parallelOps, static_cast<bool>(setDeviceParallelOps_)) != ConfirmationButton::accept)
         return;
diff -Naur -x '*.orig' -x '*.rej' -x '*.git*' 11.4-0/FreeFileSync/Source/ui/small_dlgs.cpp 11.4-1/FreeFileSync/Source/ui/small_dlgs.cpp
--- 11.4-0/FreeFileSync/Source/ui/small_dlgs.cpp	2020-12-08 08:15:29.432156507 -0500
+++ 11.4-1/FreeFileSync/Source/ui/small_dlgs.cpp	2020-12-08 19:07:22.970153675 -0500
@@ -370,11 +370,14 @@
 
     m_spinCtrlConnectionCount->SetValue(parallelOps);
 
-    m_spinCtrlConnectionCount->Disable();
-    m_staticTextConnectionCountDescr->Hide();
+    if (!canChangeParallelOp)
+    {
+        m_spinCtrlConnectionCount->Disable();
+        m_staticTextConnectionCountDescr->Hide();
 
-    m_spinCtrlChannelCountSftp->Disable();
-    m_buttonChannelCountSftp  ->Disable();
+        m_spinCtrlChannelCountSftp->Disable();
+        m_buttonChannelCountSftp  ->Disable();
+    }
     //---------------------------------------------------------
 
     //set up default view for dialog size calculation
@@ -807,7 +810,7 @@
 
     targetFolder = std::make_unique<FolderSelector>(this, *this, *m_buttonSelectTargetFolder, *m_bpButtonSelectAltTargetFolder, *m_targetFolderPath,
                                                     targetFolderLastSelected, sftpKeyFileLastSelected, nullptr /*staticText*/, nullptr /*wxWindow*/, nullptr /*droppedPathsFilter*/,
-    [](const Zstring& folderPathPhrase) { return 1; } /*getDeviceParallelOps*/, nullptr /*setDeviceParallelOps*/);
+    [](const Zstring& folderPathPhrase) { return 5; } /*getDeviceParallelOps*/, nullptr /*setDeviceParallelOps*/);
 
     m_targetFolderPath->setHistory(std::make_shared<HistoryList>(folderHistory, folderHistoryMax));
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.git*' 11.4-0/FreeFileSync/Source/ui/sync_cfg.cpp 11.4-1/FreeFileSync/Source/ui/sync_cfg.cpp
--- 11.13-0/FreeFileSync/Source/ui/sync_cfg.cpp	2021-08-17 19:18:02.039992429 -0400
+++ 11.13-1/FreeFileSync/Source/ui/sync_cfg.cpp	2021-08-17 19:33:01.693071327 -0400
@@ -348,7 +348,7 @@
 commandHistoryOut_(commandHistory),
 globalPairCfg_(globalPairCfg),
 localPairCfg_(localPairCfg),
-    enableExtraFeatures_(false),
+    enableExtraFeatures_(true),
 showMultipleCfgs_(showMultipleCfgs)
 {
     setStandardButtonLayout(*bSizerStdButtons, StdButtons().setAffirmative(m_buttonOkay).setCancel(m_buttonCancel));