@systemparadox The 'over time' slowdown appears related to the GUI actually, as hitting the icon in the upper-right and just hitting 'OK' to restart the GUI thread speeds things right back up again as a workaround at least. But yeah there's some kind of bug causing it to slow down over time. Happens with both the Legacy/SVG and the newer WebGL GUI.
Also instead of worrying about >5 ticks/second just manually upgrade the RCL and place a mixture of completed extensions and construction points. Once you know the basic code loop works, skip over the 'grindy' bits in the Simulation mode.