tl;dr - If you upgraded to SecureCRT 8.1.0 and your sessions are slow, globally change your font in all your sessions.
SecureCRT is hands-down my favorite secure terminal application on OS X, Windows & iOS. I've been using it since early 1998, and I have never found a better tool. That said...
This past weekend I got around to renewing my license so I could upgrade to SecureCRT 8.1.0. I even ponied up for a three year license / upgrade plan.
Withing a few minutes of upgrading, I knew something was wrong. Slow screen scrolling, slow pasting into sessions, absolutely intolerably slow trying to scroll back through my terminal buffer.
It was downright painful. So painful that I pulled version 8.0.2 out of my trash and ran it side by side. Tests using slo-mo video mode on my iPhone revealed that the new version was scrolling text at 1/5 of the speed of the old version. That's a big steaming pile of no joy.
While I had no crash dumps, or forensic evidence of the issue, I shot off an email to support to let them know about the issue. I got a prompt reply, as I always do from VanDyke Software. Support was wonderfully patient with me. (If you have heard that doctors make the worst patients, then ya gotta figure that DevOps Security types make the worst customers of tech.) They hadn't encountered the issue, and weren't able to reproduce; but the back and forth willingness to keep working at the issue is one of the things that makes VanDyke Software an awesome company.
While trying to document a couple of different permutations of the issue, I stumbled onto the cause. In doing so, I also realize why the ubergeeks at VanDyke Software were not able to reproduce the issue. I've been running SecureCRT on OS X since version 6.6, and some of my existing session configs were originally created in that version, back in 2010. Who knows how many bits of my configs are functional, but not optimal. One of those non-optimal settings was apparently my font. When I globally changed my font, all my sessions sped up. But wait, there's more... When I then globally changed my font back to the original setting, my sessions were still gloriously fast. How's that? I diffed one of my old session configs against a newly 'fixed' config. Despite them both having the same font selected in the GUI they had slightly different font settings in the config. Something had changed in my font catalog, and while using the original data technically worked there was a noticeable increase in overhead to make it work. Functional, but not optimal.
If this helped you out, please let me know.