One way to conserve system resources is to configure idle time-out
settings for the worker processes in an application pool. When these
settings are configured, a worker process will shut down after a
specified period of inactivity. The default value for idle time-out is
20 minutes.
If you have a just a few sites on your server and you want them to
always load fast then set this to zero. Otherwise, when you have 20
minutes without any traffic then the app pool will terminate so that
it can start up again on the next visit. The problem is that the first
visit to an app pool needs to create a new w3wp.exe worker process
which is slow because the app pool needs to be created, ASP.NET or
another framework needs to be loaded, and then your application needs
to be loaded. That can take a few seconds. Therefore I set that to 0
every chance I have, unless it’s for a server that hosts a lot of
sites that don’t always need to be running.
Idle Timeout is if no action has been asked from your web app, it the process will drop and release everything from memory
Recycle is a forced action on the application where your processed is closed and started again, for memory leaking purposes and system health
The negative impact of both is usually the use of your Session and Application state is lost if you mess with Recycle to a faster time.(logged in users etc will be logged out, if they where about to "check out" all would have been lost" that's why recycle is at such a large time out value, idle timeout doesn't matter because nobody is logged in anyway and figure 20 minutes an no action they are not still "shopping"
The positive would be get rid of the idle time out as your website will respond faster on its "first" response if its not a highly active site where a user would have to wait for it to load if you have 1 user every 20 minutes lets say. So a website that get his less then 1 time in 20 minutes actually you would want to increase this value as the website has to load up again from scratch for each user. but if you set this to 0 over a long time, any memory leaks in code could over a certain amount of time, entirely take over the server.
I have inherited a desktop app that makes calls to a series of Web Services on IIS. The web services (also) have to be able to run timed processes, independently (without having the client on). Hence they all have timers.
The web service timers were shutting down (memory leak?) so we set the Idle time out to 0 and timers stay on.