We have the same issus. First my computer has 8GB memory. After one or two hours of working with the old composer its become very slow so I close the browser and reopen it. Than I get an upgrade to 16GB an its work a little bit better. The memory upgrade seems not so efficient because the browser (firefox) is limit and use (I think) maximum 4 GB.
So at the moment I show the mashups in chrome and do my composer stuff with firefox. That has the advantage that the refreshing of the frontend doesn't blog the composer.
What does cut in pieces mean? I was under the impression that the whole point of thingworx is it's ability to bring many different services together and add them to a mashup.
We have the same slowness but it is unusable when it gets this slow. Clicking on a chart to edit it takes 10 seconds for the screen to accept the click.
My computer has a xeon processor and 32GB of RAM and the composer is unusable after 5 min.
Hard to answer without seeing what you had done, but on our case it works well, and as any other tool and technology there's always performance tuning.
About the old composer beeing unresponsive a while ago I had problems with Chrome extensions (LastPassword) I don't know why but they affected, I usually open TW development on another Chrome instance and I don't have issues - well I never close my computer and sometimes need to restart the tab :P -