Ramin,
9.1 was released specifically to fix performance issues. We fixed Issues with JSPs that could take minutes to open (now open in seconds) and JSP and XHTML files where typing was slow and even clicking could take time. JavaScript validation load for JS contained in JSP files and JS across the workspace was also significantly reduced.
Performance problems can be caused by a variety of reasons, so without knowing the exact nature of your problem, I cannot say for sure that it is fixed in 9.1; however, there is a good chance that it is, given the performance related fixes that were a primary part of 9.1.
You can install 9.1 in parallel with 8.6.1, I would encourage you to try it out and let me know if your issues are fixed. If not, we can work on your specific problems for the next release.
I appreciate your continued patience and support.