Yep thx eservent for your feedback....
For us the cache is not a solution because when a User launch the XaaS all datas are differents for each of them.
The only solution is to execute the action asynchronously but the answer of vMware support is :
Unfortunately, that is not possible. The load is done in order from the first one to the last one.
I have even asked our development team and they said that this is not possible.
However, I have find a few blogs that have some guides to optimize workflows:
https://bluemedora.com/optimize-workflows-vra-vro/
http://federicocinalli.com/blog/item/218-starting-with-vmware-vrealize-orchestrator
https://blog.purestorage.com/documentation-update-best-practices-and-vrealize/