The web editor is sluggish (in Firefox) after the latest update. I suspect this is because of the on-the-fly DTD validation. Chrome does not seem to be affected.
FWIW, this is much easier to see as the topic gets larger. For really large topics the delay for keystrokes to render (or other actions) can be a few seconds. Perhaps having a validate button would be better in this case, so that way it's less background work, as at the moment we have three core components working in the background (spelling, validation and rendering). The other option that springs to mind is a toggle to turn the validation on or off. I'm not sure how valid these options are though as I'm not familiar with the specific implementation.
Fixed in 201306191339 The spell checking is now done in a web worker. Firefox is still slower than chrome (copying data back and forth between the ui thread and the spell checking thread is slow in Firefox), but you don't get the freezes in large topics. NOTE: When QAing bug fixes, please make sure the build you are using is equal to or higher than the build identified above. Be aware that this fix may have only been applied in PressGang Next at this time, or it may not be currently applied in any production system, in which case it can only be verified at a later date.
Moving this bug to CLOSED CURRENT_RELEASE to clean up old bugs that were QA'd by the PressGang team but not by the original reporter. If the bug is still present then please re-open the bug.