Does a computer not rebooting get slower if there is no page outs?

Not necessarily. Time or use doesn't make a computer slower. Your computer's performance is directly impacted by what you have running at any point in time, and somewhat by your hardware configuration.

Again, only sometimes. It depends on what you had running before and what you have running after you restart. Many Mac users run for months between restarting, with no performance degradation at all.

To determine if you can benefit from more RAM, launch Activity Monitor and click the System Memory tab at the bottom to check your page outs. Page outs are cumulative since your last restart, so the best way to check is to restart your computer and track page outs under your normal workload (the apps, browser pages and documents you normally would have open). If your page outs are significant (say 1GB or more) under normal use, you may benefit from more RAM. If your page outs are zero or very low during normal use, you probably won't see any performance improvement from adding RAM.

Mac OS X: Reading system memory usage in Activity Monitor

If you're having performance issues, this may help:

Performance Tips For Mac OS X​

 

Again, not necessarily. Uptime does not make a computer slower.

Again, it depends on what you have running, how much drive space you have available, what apps/widgets/processes you have running, even the quantity of fonts you have installed. There are many factors that contribute to performance. It's not just RAM, and you don't have to restart to free up memory. Mac OS X automatically manages memory, without the need to restart. Even if you're paging out, as soon as the high memory demands subside, memory is made available to other apps. It's not like Windows used to be.

 

One of mine has been on for over a year. 834 MB of page-ins, 212 KB of page-outs. For the entire 383 days. It's a Macmini1,1 with 2 GB of RAM, and is primarily a server, though it occasionally does other things. And at 383 days uptime, its no slower now than a year ago.

 

Yes, I've both heard such stories and experienced it myself, with earlier versions of Windows, but not with Mac OS X. With earlier Windows versions, you had to restart to free up RAM after running some memory-intensive apps. That isn't necessary with Mac OS X. If performance slows after time, it's not a result of uptime since the last restart. It's due to other factors. Many will attribute it to uptime because instead of diagnosing what's really causing the slow performance, they restart, which shuts down the app or process that may have been the culprit. They think the restart sped things up, when in fact, they could have achieved the same result without restarting.