Why is Tor Slow?
Users often complain that the Tor network is slow or has inconsistent speed. This page briefly describes some reasons for affected Tor throughput. Interested readers can also refer to the Tor Project FAQ and relevant research for a more detailed explanation.
Factors Affecting Tor Throughput
Misuse of the Tor Network
Some actors misuse the Tor network, either purposefully or due to a lack of knowledge. For instance, Tor is sometimes used to conduct DDoS attacks. By doing this, the Tor relays are the ones who actually suffer from the attack, instead of the intended target. Some people use peer-to-peer software (like BitTorrent) through Tor which slows down the network for all users. 
Tor relays are run by volunteers  in a decentralized way. Consequently, relays do not have uniform quality; some are big and fast, while others are smaller and slower. As a whole, the network could be faster if it had more capacity. To improve the capacity of the Tor network, users can either run a Tor relay or help existing relays.
Tor Circuits Lengthen Connections
When navigating to clearnet resources, Tor provides anonymity by building circuits with three relays. So instead of connecting directly to the destination server, a connection is made between each relay of the circuit and this takes more time. In the case of onion services, a six-relay arrangement is used in the connection - three picked by the user and three picked by the onion service.
In addition to using multiple relays, Tor tries to build circuits with relays in different geographical locations. This necessarily causes connections to travel further and slows down the fetching of resources.
- Directory Information Download Overhead: Users with low bandwidth (like those on cell phones) have to spend too much time downloading directory information. Tor protocols need to be optimized for efficiency.
- Excessive User Load: Some users put excessive traffic load on the Tor network relative to their network contributions. Methods of limiting these effects and prioritizing other users need to be implemented. 
- Tor Congestion Control: Tor's mechanism does not work well in combining high-volume (bulk transfer) and low-volume (browsing) streams.
- Tor Latency Failures: Tor is inefficient in handling connection failures or high / variable latency. Better heuristics to move away from bad circuits and a more uniform latency response is required.
- Tor Load Distribution: Tor's current path selection algorithms do not effectively distribute the network load. The properties of relays need to be more accurately estimated so relays do not become over or under-loaded. 
- Tor Network Capacity: As noted earlier, the total capacity of the Tor network is insufficient relative to unmet privacy demand. A significant boost in the overall number of relays is required. 
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- A large file downloaded through BitTorrent can translate to several hours of browsing for the regular Tor Browser user.
- And hostile actors.
- Roger Dingledine is the co-creator of the first alpha version of Tor.
- As the research is dated, some of these issues may have been fully or partially mitigated by now.
- This may involve targeting specific user profiles (e.g. throttling certain protocols) so the original Tor design of high throughput and good latency properties can be realized.
- Capacity is currently estimated by observing the largest traffic burst seen in the past day. This bandwidth capacity is advertised in the directory information, leading clients to preference their path selection based upon a relay's estimated bandwidth.
- Economics suggests increased supply will lead to more users arriving to fill the void.