How do I speed up the decoding process?

[NightshadeNightshade]

I'm using Avidemux 2.7 - 64 bits, and I'm downsizing 4k MKVs, which average around 40BG in size, to 20GB.
Right now, it takes around 15 hours if I use the "Video Size (Two Pass)" option and 8-to-10 hours if I ball-park it with "Constant Rate Factor (Single Pass)".
Is there a way to speed the decoding process up, without loosing additional quality in the decoded end MKV?
Thanks.

[video.baba]

I'm downsizing 4k MKVs

So that's a massive load of data.

it takes around 15 hours if I use the "Video Size (Two Pass)

I'm really curious as to why you would need to do that. Do you only have a specific amount of space on the output-drive for a video so you need to compromise quality for size?

I don't know the program you are using, however...
You don't say what video codec you are encoding to... We can't read your mind!
Do you have any filters or tunes set? Post-processing?

[KarMa]

You can use LAV Filters (GPU decoding) to decode video for Avisynth, but that comes with the baggage of having to use Avisynth. I'm not sure of any easy to use programs that have GPU decoding options. FFMPEG has GPU decoding support but I'm not entirely sure on all the commands but FFMPEG would be easier to get into than Avisyth. You should also mention what CPU and GPU you have.

[gonca]

Handbrake might have GPU ENcoding, which I think is what you want
That would speed up your encodes
You might be able to use VidCoder (preferred) as well

You will need a NVidia GPU

[andkar]

Are you sure it's the decoding that's the bottleneck?

[pandy]

I'm using Avidemux 2.7 - 64 bits, and I'm downsizing 4k MKVs, which average around 40BG in size, to 20GB.
Right now, it takes around 15 hours if I use the "Video Size (Two Pass)" option and 8-to-10 hours if I ball-park it with "Constant Rate Factor (Single Pass)".
Is there a way to speed the decoding process up, without loosing additional quality in the decoded end MKV?
Thanks.

Use temporary file with some lossless codec - it will reduce CPU use at a cost some space.

[KarMa]

Are you sure it's the decoding that's the bottleneck?

Software decoding of 4K content which is then downsize to 1080p (I'm assuming that's what OP is doing), would probably be decoding bottle necked.

[johns0]

Use vidcoder,if that doesn't help get a better computer.