diff --git a/aegisub/src/threaded_frame_source.cpp b/aegisub/src/threaded_frame_source.cpp
index 0225161c6..a362098c1 100644
--- a/aegisub/src/threaded_frame_source.cpp
+++ b/aegisub/src/threaded_frame_source.cpp
@@ -123,6 +123,7 @@ void *ThreadedFrameSource::Entry() {
 		double time;
 		int frameNum;
 		std::unique_ptr<AssFile> newSubs;
+		std::deque<std::pair<size_t, std::unique_ptr<AssEntry>>> updates;
 		{
 			wxMutexLocker jobLocker(jobMutex);
 
@@ -138,11 +139,27 @@ void *ThreadedFrameSource::Entry() {
 			frameNum = nextFrame;
 			nextTime = -1.;
 			newSubs = move(nextSubs);
+			updates = move(changedSubs);
 		}
 
-		if (newSubs) {
+		if (newSubs || updates.size()) {
 			wxMutexLocker fileLocker(fileMutex);
-			subs = move(newSubs);
+
+			if (newSubs)
+				subs = move(newSubs);
+
+			if (updates.size()) {
+				size_t i = 0;
+				auto it = subs->Line.begin();
+				// Replace each changed line in subs with the updated version
+				for (auto& update : updates) {
+					advance(it, update.first - i);
+					i = update.first;
+					subs->Line.insert(it, *update.second.release());
+					delete &*it--;
+				}
+			}
+
 			singleFrame = -1;
 		}
 
@@ -194,13 +211,27 @@ ThreadedFrameSource::~ThreadedFrameSource() {
 	Wait();
 }
 
-void ThreadedFrameSource::LoadSubtitles(AssFile *subs) throw() {
-	subs = new AssFile(*subs);
+void ThreadedFrameSource::LoadSubtitles(const AssFile *subs) throw() {
+	AssFile *copy = new AssFile(*subs);
 	wxMutexLocker locker(jobMutex);
 	// Set nextSubs and let the worker thread move it to subs so that we don't
 	// have to lock fileMutex on the GUI thread, as that can be locked for
 	// extended periods of time with slow-rendering subtitles
-	nextSubs.reset(subs);
+	nextSubs.reset(copy);
+	changedSubs.clear();
+}
+
+void ThreadedFrameSource::UpdateSubtitles(const AssFile *subs, std::set<const AssEntry*> changes) throw() {
+	std::deque<std::pair<size_t, std::unique_ptr<AssEntry>>> changed;
+	size_t i = 0;
+	for (auto const& e : subs->Line) {
+		if (changes.count(&e))
+			changed.emplace_back(i, std::unique_ptr<AssEntry>(e.Clone()));
+		++i;
+	}
+
+	wxMutexLocker locker(jobMutex);
+	changedSubs = std::move(changed);
 }
 
 void ThreadedFrameSource::RequestFrame(int frame, double time) throw() {
diff --git a/aegisub/src/threaded_frame_source.h b/aegisub/src/threaded_frame_source.h
index a6080d25d..62b101793 100644
--- a/aegisub/src/threaded_frame_source.h
+++ b/aegisub/src/threaded_frame_source.h
@@ -19,7 +19,9 @@
 /// @ingroup video
 ///
 
+#include <deque>
 #include <memory>
+#include <set>
 
 #include <wx/event.h>
 #include <wx/thread.h>
@@ -28,6 +30,7 @@
 #include <libaegisub/scoped_ptr.h>
 
 class AegiVideoFrame;
+class AssEntry;
 class AssFile;
 class SubtitlesProvider;
 class VideoProvider;
@@ -46,6 +49,7 @@ class ThreadedFrameSource : public wxThread {
 	int nextFrame;   ///< Next queued frame, or -1 for none
 	double nextTime; ///< Next queued time
 	std::unique_ptr<AssFile> nextSubs; ///< Next queued AssFile
+	std::deque<std::pair<size_t, std::unique_ptr<AssEntry>>> changedSubs;
 
 	/// Subtitles to be loaded the next time a frame is requested
 	std::unique_ptr<AssFile> subs;
@@ -70,7 +74,15 @@ public:
 	///
 	/// This function blocks until is it is safe for the calling thread to
 	/// modify subs
-	void LoadSubtitles(AssFile *subs) throw();
+	void LoadSubtitles(const AssFile *subs) throw();
+
+	/// @brief Update a previously loaded subtitle file
+	/// @param subs Subtitle file which was last passed to LoadSubtitles
+	/// @param changes Set of lines which have changed
+	///
+	/// This function only supports changes to existing lines, and not
+	/// insertions or deletions.
+	void UpdateSubtitles(const AssFile *subs, std::set<const AssEntry *> changes) throw();
 
 	/// @brief Queue a request for a frame
 	/// @brief frame Frame number
diff --git a/aegisub/src/video_context.cpp b/aegisub/src/video_context.cpp
index 7ec26e3df..325ee4bc5 100644
--- a/aegisub/src/video_context.cpp
+++ b/aegisub/src/video_context.cpp
@@ -233,10 +233,13 @@ void VideoContext::Reload() {
 	}
 }
 
-void VideoContext::OnSubtitlesCommit() {
+void VideoContext::OnSubtitlesCommit(int type, std::set<const AssEntry *> const& changed) {
 	if (!IsLoaded()) return;
 
-	provider->LoadSubtitles(context->ass);
+	if (changed.empty())
+		provider->LoadSubtitles(context->ass);
+	else
+		provider->UpdateSubtitles(context->ass, changed);
 	if (!IsPlaying())
 		GetFrameAsync(frame_n);
 }
@@ -446,7 +449,7 @@ void VideoContext::LoadTimecodes(wxString filename) {
 		ovrFPS = agi::vfr::Framerate(STD_STR(filename));
 		ovrTimecodeFile = filename;
 		config::mru->Add("Timecodes", STD_STR(filename));
-		OnSubtitlesCommit();
+		OnSubtitlesCommit(0, std::set<const AssEntry*>());
 		TimecodesOpen(ovrFPS);
 	}
 	catch (const agi::FileSystemError&) {
@@ -469,7 +472,7 @@ void VideoContext::SaveTimecodes(wxString filename) {
 void VideoContext::CloseTimecodes() {
 	ovrFPS = agi::vfr::Framerate();
 	ovrTimecodeFile.clear();
-	OnSubtitlesCommit();
+	OnSubtitlesCommit(0, std::set<const AssEntry*>());
 	TimecodesOpen(videoFPS);
 }
 
diff --git a/aegisub/src/video_context.h b/aegisub/src/video_context.h
index a9cd637ea..37e4e4fa2 100644
--- a/aegisub/src/video_context.h
+++ b/aegisub/src/video_context.h
@@ -32,10 +32,10 @@
 /// @ingroup video
 ///
 
-#include <time.h>
-
+#include <ctime>
 #include <list>
 #include <memory>
+#include <set>
 
 #include <wx/timer.h>
 #include <wx/stopwatch.h>
@@ -45,6 +45,7 @@
 #include <libaegisub/vfr.h>
 
 class AegiVideoFrame;
+class AssEntry;
 class SubtitlesProviderErrorEvent;
 class ThreadedFrameSource;
 class VideoProvider;
@@ -139,7 +140,7 @@ class VideoContext : public wxEvtHandler {
 	void OnVideoError(VideoProviderErrorEvent const& err);
 	void OnSubtitlesError(SubtitlesProviderErrorEvent const& err);
 
-	void OnSubtitlesCommit();
+	void OnSubtitlesCommit(int type, std::set<const AssEntry *> const& changed);
 	void OnSubtitlesSave();
 
 	/// Close the video, keyframes and timecodes