Also, if you're curious about bandwidth limitations in this particular cloud, take a look at these benchmarks:
<a href="http://viktorpetersson.com/2012/01/23/benchmarking-virtual-network-drivers-under-freebsd-9/" rel="nofollow">http://viktorpetersson.com/2012/01/23/benchmarking-virtual-n...</a>
<a href="http://viktorpetersson.com/2012/01/24/benchmarking-and-tuning-freebsds-virtio-network-driver/" rel="nofollow">http://viktorpetersson.com/2012/01/24/benchmarking-and-tunin...</a>
(Using CloudSigma as the cloud-vendor and FreeBSD 9 as the host)
I might be missing something due lack of experience with MongoDB, but what did disk I/O have to do with the secondaries falling behind, and why did effectively eliminating one of the secondaries "fix" that? The description makes me think there's a pathological case lurking in MongoDB that needs to be looked at more closely.