Actually the above didn’t solve the issue after all. I thought it had but actually not.
I then removed all packages (except for the package manager) and the problem persisted. Using top I could then see that there was very obviously a memory leak. The application started out with 20% cpu usage and about 20MB of RAM usage. Watching top (with no files open and a blank new window of Sublime open only) I could see the RAM usage steadily ramping up by about 20MB per second… and it just kept going up till it maxed out at about 8GB and the CPU usage at about 400%. It also spawned a second process.
I then tried deleting the data folder (i.e. ~/Library/Application Support/Sublime Text 3) and watched top again. After that it seemed to be behaving… top showed RAM usage as stable @ 49MB
Next I opened a project. I initially got 2 Sublime processes in top; one with 49MB and the other with about 200MB… after about 30 seconds they seemed to combine into a single process with about 330MB RAM usage (which seems a bit extreme but at least it’s not ramping up).