rmf files generally aren't portable from one version of WC to the next... Trying to do so generally makes the file un-usable in any version. Try WC 3.3 ? It's about as close as to hammer as it gets, and I find its more reliable then any version of hammer or WC.

Use the map file when trying to port the map to another version of WC.

? posted by ReNo

You definately installed 3.5 beta right yes? You know it has to be installed over the top of 3.4, by just replacing the old .exe with the new? I'm sure you did that, but its worth checking If you want you can send me the rmf and I'll try loading it on my computer, to see if its just something at your end. reno@elva-mod.co.uk

I have 3.4 and 3.5 in the same folder named differently and they both work just fine. I can switch between the two if need be. You do not have to overwrite the .exe.

There should not be issues with porting a map between 3.3 and up. I've done it myself plenty of times with complex stuff, the problem lies somewhere else.

I'd still try my suggestion which tailors on the same principle as yak_fighter's which is that you might have too much showing at once. Try scrolling your 2D windows in so that it shows a much smaller window, you'd be surprised at how it will respond better once your map gets larger. (Even on a higher class computer than what he said was used.)

Whether it works or not as far as your eyes can see, its still probably unwise to do. Hammer 3.4 comes with numerous files that 3.5 does not, such as sprites to use in editor (such as the lightbulb, speaker, etc...), prefabs, help files, and so on. Surely you are missing all of these things because you just use the single executable files that comes with 3.5?

... maybe thats because... its saving the map? That will generally slow most programs down...