How to make xvid keyframes
Things you need:

1. Install Avisynth

2. SCXviD.dll
3. ffms2.dll

Place these in the Avisynth\Plugins folder wherever you just installed it.

4. AVSMeter

Put the exe in a system folder, like C:\Windows.
If you know what you're doing, you can put it in any folder and add it to your path.
You could also put it in whatever folder you run the batch from.

5. This batch script (below if you don't want to download it)

@echo off
echo Making SCXvid keyframes...
set video=%~1
set video2=%~n1
echo FFvideosource("%video%") > "%video2%_keyframes.avs"
echo SCXvid("%video2%_keyframes.log") >> "%video2%_keyframes.avs"
avsmeter "%video2%_keyframes.avs"
del "%video2%_keyframes.avs"
del "%video%.ffindex"
echo Keyframes complete

Paste this into a text file and rename the file to keyframes.bat (or whatever.bat you want).
If you want to have all keyframes in the same folder, change the scxvid line to
echo SCXvid("D:\your_path_here\%video2%_keyframes.log") >> "%video2%_keyframes.avs"
You can drag your video file onto the batch script, or you can open cmd.exe in the video's folder and type:
keyframes "your_video.mkv"
where keyframes is the name of your batch file and the other thing is your video no matter what the extension.
You don't need the quotation marks if there's no space in the filename.
You'll see progress as it encodes. When done, go to "Video - Open Keyframes" in Aegisub and select the .log file.
If you're in a hurry, you can load it while it's being generated, and it will load whatever's done so far.
Don't forget to reload it before you time the part that wasn't done before.

SCXvid keyframe generation script v2.0 (This is supposedly faster, but it doesn't work for me, so feel free to try.)

1. Download standalone SCXvid here
2. Download FFmpeg here
3. Create batch script below:

@echo off
echo Making SCXvid keyframes...
set video=%~1
set video2=%~n1
ffmpeg -i %video% -f yuv4mpegpipe -vf scale=640:360 -pix_fmt yuv420p -vsync drop - | SCXvid.exe %video2%_keyframes.log
echo Keyframes complete

4. Run batch script and call your video file.

Only works with 4:2:0 video, but this shouldn't be a problem 99% of the time.