even if you note that you have made improvements in handling large files with R# 5.1.2 release this seems not to be the case for VB sources. I'm working under Visual Studio 2008 on a VB solution. The application is an old legacy one which was converted from VB6. When i try to edit large files, the class file i'm currently working on has aroun 11.000 lines, the edit performance decrease more and more, specially when there are many compiler errors (I accidentially pressed Ctrl-K-U twice because of the slow performance and therefore all the comments in the former uncommented code block where treated as normal code). CPU activity of VS is very high and the cursor in the editor window don't react for seconds. When i take a look at the VS instance in the process explorer at this time i see that VS is busy doing garbage collection (60% .. 90%).
I notice that VS becomes slow with all R# releases except R# 4.5 when i work on a VB solution with large fiels and / or many compile errors.