## 1 Introduction  
The Internet increasingly permeates everyday life of individuals around the world. On the other hand, information technology allows data analysis to a degree which was inconceivable a few years ago. Simultaneously to the increasing amount and availability of information about Internet users, new information retrieval, data mining and further technologies allow to automatically collect, filter and analyze personal information and to draw conclusions based on this process. Democratic societies should further advocate environments that respect user privacy for citizens and should support individuals who face repressive censorship to access public information without being identified or traced. In political regimes, where personal rights, the freedom of speech, and in particular free access to information is restricted, these possibilities of modern data collection can lead to persecution of individuals if their identity is unveiled. Another restraint is censorship, which may be used to restrict free access to information [1].  
By using anonymization tools such as the Tor onion routing network [2, 3], Internet users can conceal their virtual tracks to a certain degree by obfuscating their IP addresses, allowing for a more anonymous Internet access. With Tor, application messages are not directly routed to the receiver, but are encrypted and forwarded through ephemeral paths of Tor relays through an overlay network, using more complicated routes that are difficult to analyze for third parties. The more users participate, the harder it is to correlate senders and receivers, and the less likely it is for any user to raise suspicions simply by using Tor: \"Anonymity loves company\" [4]. The anonymity provided within the Tor network attracts many different groups of users, such as journalists and activists or business, governmental, military, and private users [5, 6]. A recent study showed significant growth of Tor users in China as the governmental censorship increased and also in Iran when the riots after the presidential election took place [7].  
However, due to usability problems caused by Tor, many \"average\" Internet users refrain from using it. This causes a negative impact on the potential overall privacy provided by Tor, since it depends on the size of the user community and availability of shared resources such as Tor relays. Besides one-time installation and configuration efforts, the main usability loss when using an anonymization tool such as Tor is an increase in latency. Several authors already discussed technically why Tor is slowing down a client's Internet speed and proposed how to improve the performance [7, 8, 9]. However, detailed comparative measurements about the performance of Tor are crucial for assessing and solving this issue.  
This paper presents distributed measurements on Tor latency and throughput for surfing to 500 popular websites from several PlanetLab nodes around the world during the period of 28 days (PlanetLab [10] provides a globally distributed testbed for network research). Furthermore, we compare these measurements to critical latency thresholds gathered from web usability research, including our own user studies. The resulting expected user cancelation rate--_i.e._, the percentage of users who abandon the wait during a certain time interval--is an indicator how easy it would be to keep existing users and to attract new, \"average\" Web users to Tor for increasing their own anonymity as well as the anonymity of the whole user community. Our results could also be relevant for integrating similar privacy-enhancing technologies into a \"Future Internet\".  
The structure of the paper is the following. We present related work in Section 2, followed by a description of our measurement setup in Section 3. The experimental results are presented in Section 4. An interpretation of those results from the perspective of web usability is given in Section 5. Section 6 discusses limitations and future work. Section 7 concludes the paper.