True, this is a limitation of Blenders compositor. It can take a few minutes to composite the full node tree.

First go to is to 'solve' your image digitally. This site can do that for you: http://magiceye.ecksdee.co.uk/

Next, try a different noise image if you are using a custom one - perhaps make it bigger in scale to increase the 'noisiness'.

Last - the 'slice multiplier' and 'slice width multiplier' parameters need tweaked for every new image - this can take a while.

The reddit page

https://www.reddit.com/r/MagicEye/

and their help guide may be of use

https://www.reddit.com/r/MagicEye/comments/i32l64/dont_know_how_to_view_magiceye_autostereograms/

Be patient, once you get it, it feels awesome.