0.99.5.2 contains a critical performance fix following a regression that was introduced in 0.99.5. Affected servers will have experienced increased CPU and RAM usage with a knock on effect of generally sluggish performance.
Separately, we are also looking into reports relating to further performance degradations that may have been introduced as part of 0.99.5, though consider the 0.99.5.2 fix to be a significant improvement on previous 0.99.5.x releases.
Please upgrade asap.
You can get the new update here or any of the sources mentioned at https://github.com/matrix-org/synapse. Note, Synapse is now available from PyPI, pick it up here. Also, check out our Synapse installation guide page.