Maybe it’s not a matter older versions of Renoise running better, but that your new songs use more CPU intensive instruments, effects, and meta devices?
Try using deprecated devices, and less of them, for example. You can also try converting XRNS to XM but you will lose a lot in the process.
If they were the same, they would play the same… You’re doing something different IMHO.
Seems to me like a matter of disciplining yourself to write low CPU songs?