On the higher-end server running osm2pgsql with a 8G cache, the speedup ratio is at least 2.1 and seems to increase with the size of the diff. Considering the very few samples we have, it’s hard to tell the effect of the cache size increase. But the fact is that this higher-end server behaves better, a speedup factor between 2 and 2.75 can reasonably be expected. This would translate to a daily server downtime of around 4h for planet updates. Details follow.
For the Oct 1 -> 2 incremental diff, current server (1.5G cache) = 555mn, higher end server (8G cache) = 262mn, speedup factor = 2.12.
For the Oct 2 -> 3 incremental diff, 587 vs. 223mn, speedup factor = 2.63
For the Oct 3 -> 4 incremental diff, 628 vs. 229mn, speedup factor = 2.74
For the Oct 4 -> 5 incremental diff, 598 vs. 220mn, speedup factor = 2.72