I know ALM has known issues that result in complex processes (especially processes that use more than a few conditions) to take inordinately long to execute. One of my colleagues has proposed a modification to our Incident process that would require the addition of 20 decisions hanging off a single Add Note action and 20 Assignment actions, one for each decision. It's an ugly hack but the elegant solution would require additional links with ALM and I won't have time to set it up for another few months.
Before I approve the hack I wanted to see whether anyone knew of performance concerns as process complexity rises. The process has just over 100 elements right now.
The number of decisions in a row can be a problem, depending on the version you have. Somtimes you cannot save the process if you have too many. If the process saves, I haven't issues running the result, but I'd certainly advise seeing if your version can save the string of decisions first.