As a developer, my customers rely on constant connectivity to Salesforce.com and therefore need to know what the SLA is for salesforce.com services in the event of a service disruption? Is there a maximum outage period committed to by Salesforce?
Not exactly what you asked for, but have a look at http://trust.salesforce.com/trust/status/ to see historical server status.
From the Master Service Agreement as at May 2016
3.1. Provision of Purchased Services. We will (a) make the Services and Content available to You pursuant to this Agreement and the applicable Order Forms, (b) provide applicable SFDC standard support for the Services to You at no additional charge, and/or upgraded support if purchased, (c) use commercially reasonable efforts to make the online Services available 24 hours a day, 7 days a week, except for: (i) planned downtime (of which We shall give advance electronic notice as provided in the Documentation), and (ii) any unavailability caused by circumstances beyond Our reasonable control, including, for example, an act of God, act of government, flood, fire, earthquake, civil unrest, act of terror, strike or other labor problem (other than one involving Our employees), Internet service provider failure or delay, Non-SFDC Application, or denial of service attack.
From Proven reliability
Our track record speaks for itself: Salesforce1 Platform has a proven 99.9+ percent uptime record for years. To ensure maximum uptime and continuous availability, Salesforce1 Platform provides redundant data protection and the most advanced facilities protection available, along with a complete data recovery plan.
You should contact your AE [Account Executive] for their official uptime document.
Answers from third parties
From Top 9 Reasons Why You Should Switch from Salesforce.com to Salesboom.com. (Could be heavily biased)
[Salesforce] has never taken steps to offer a service guarantee
From CRM Online VS SalesForce.com (again, probably biased but potentially correct)
NO SERVICE LEVEL AGREEMENT (SLA)
Salesforce.com does not offer a service level agreement.
What regarding performances degradation?
On trust.salesforce.com it is explicitly declared that if the average of the transactions goes over 0.5 for 2 minutes consecutively, this is considered a performance degradation in the current status, and if it is for 10 minutes consecutively is a performance degradation tracked in the historical section.
Is there any SLA about Performance degradation?