As I understand it, both conditions and actions increase server load (as well as anything that needs to be continuously reevaluated from the script [HUD texts, effects, camera, etc.] and anything non scripted that happens in the game [default hero abilities, healing and damage, deaths, objectives, and player movement] as well as the AI of the default AI bots that can be added in the lobby).
Conditions in ongoing rules are checked every single frame (there is short-circuiting so the order of conditions can make a difference).
(See this post by Dan Reed:)
As for actions, they can lead to server load peaks, though if they are simple actions such as changing a variable, you can run hundreds of them in a single frame without any problems. The problems may start if the same number of actions are run for every single player in a match with 12 players and at short intervals (loops with very short wait times greatly affect server load, in my experience).
I personally try to keep the number of ongoing rules (and especially ongoing - each player rules) and the number of conditions in these rules as low as possible. From my experience, subroutines have next to no impact on server load when they are not being called.
(I’ve posted a list of some general principles I try follow when writing workshop scripts in this thread: [Workshop Modes] Delwion's Workshop Modes [at the bottom of the first post].)