You do not say the amount of hardware memory, free SSD space, size of the offending (slow) project, the length of the session since Xojo launch until you quit, etc.
And another thing is forgotten (above) is the Cancel feature: work for 10 hours, then press the Cancel shortcut for your platform (Cmd-2 or Ctrl+Z) and wait until you cancelled all of your 10 hours of work.
I really do not know how the Cancel feature (nor the Xojo IDE) is coded, but it certainly takes memory, too many memory.
What was not said too is… how many other applications run concurently to Xojo (DVB software to watch Tennis / Football / Naked woman (!), etc.) what are-you uploading or downloading, and so on.
It is slow now here too (El Capitan Xojo 2015r1): go figure… I am quite sure Xojo is not the cause / the real cause is elsewhere !
No Plug-In here, but downloads / near to full SSD / DVB (TV running on an external monitor)…
Have you made a test with a brand new comuter running only Xojo / no plugin / no internet / plenty of free room in the SSD and a 5KB project ?
Just for the sake of telling: “Xojo is slow” or “I do not code like that” (but in this case, try it, then give answers).
Regards,
Happy New Year !