How to run large amount of blocking/sync I/O operations

I have a list of around 100k users and I need to perform a sync WCF call for each of them and I need to complete the list as fast as possible. Each WCF call is around 200ms-1s depending the server status. I was thinking of using Parallel.For but is there better way?

Blocking IO is not awkward to parallelize because it ties up threads, and threads are relatively expensive. Additionally, increasing parallelism can often degrade performance if all the work is going to the same backend, as it can increase a lot of overheads at the server and hit bottlenecks.

IMO the real problem here is the size of the unit-of-work, where it sounds like you need to perform an API call per rexord; 1000 batches of (say) 100 records is usually vastly more performance than 100k single operations. If it was me, I'd be looking to refactor this into a batched API. If the underlying service can't be changed, another option is to run an orchestration service as close to the backend as possible (same box, ideally), that can at least receive batches and unroll them locally, minimizing latency overheads. Note however that if latency isn't a significant factor in the current issue, this won't help: if the 200ms-1s is simply due to a slow server implementation, then you probably can't do much to work around that (except maybe ... invest the time to make it less slow).

A secondary concern here would be async; async can be important (especially server-side), but in this instance it isn't where I'd focus.