When connecting using REST, the connector will allow the server to compress the data it sends to reduce bandwidth usage and improve performance on slower networks. Note that some servers may not support compression, or may disable it.
If you have an extremely fast network, e.g. if the server is located physically near you and you have a 1Gbps connection, disabling this compression may result in a slight improvement in performance. To disable this compression, open the task repository settings for your repository (click the repository name at the top of the task editor, OR go to Window > Show View > Other > Mylyn> Task Repositories and double click on your repository), and, under Additional Settings, uncheck the option to Enable network compression.