unfortunately h264 is not really a great codec for editing in ANY editor. Luckily hardware has gotten really fast so it can be done, but sometimes causes progam to choke.
Try to proxy your stuff to cineform or DNxHD or prores 'before' you do the speed change. Like, start from the beginning with the proxy on.
Might be a lot better.
ON card issue, sometimes using software only will do the trick but take longer ( it uses cpu only and don't think it uses any graphic chip stuff ).
Don't be mad at us imperfect users if we don't express ourselves like people who studied rhetoric at university. I recently realized I need to rein in my subjective attitude and be more objective, due to insulting some poster here.
Anyway, good luck and it shouldn't take long to try those things. You might have to delete your cache before you start from the beginning, cause you're right.... there is some sloppy programming going on with all the new additions and so on.