Alex Mohammed I think the issues here were more due to outdated PHP than slow software or server load. If you scale to that size, we can certainly support you. The recent v10b2 update has done a lot of speed things up. A site of 150k users would certainly need some individualised attention, setting up remote storage and optimizing server load would also be required at this scale.
We do have sites at close to this scale (and growing), they are mostly running well, particularly with the recent update.