Lots of converters out there. Conversion Speed is usually controlled by the system that is doing the work, such as if your system has a lightweight CPU it will take longer than on a system with a powerful CPU, additionally the Hard Drive or SSD also comes into play with conversion speed.
For example I have a system that if I do the conversions by use of my hard drive, my quadcore CPU only runs at 60-70% for all cores. But if the video to be converted is on my SSD my CPU then hits 98-100% for all 4 cores, so its more efficient and faster to convert with the SSD drive which can read in the one video and write out the other video format thats being created very fast vs a slower hard drive. Other way to speed it up involves 2 internal hard drives so that one drive contains the video to be converted and the other drive is the destination to write the newly created video in the format of choice and this too causes the CPU to run more efficiently by not being bottlenecked by the drive that the video is read and converted on.
Only other slow conversion I have seen before are trial or demo versions of converters where they throttle them down as a means to get you to register and buy a licensed copy of the converter. These converters sometimes even watermark your video which is sad but its a means for them to get you to cough up the money to buy a licensed copy of the software to then be able to convert the videos faster and without watermark.
I use a tool called VirtualDub64 which is free and fast and you can create jobs and run batched conversions, but I am not sure if it will handle mkv format or not and I cant check from here since I am at work on break and have content restrictions
. Other one would be FormatFactory which might also support mkv and is also free.
