Concurrency
How many proxy connections you may run at once — often the real constraint on throughput, not bandwidth.
Concurrency is the number of simultaneous connections a plan permits. It is frequently the limit you hit first, and frequently the one buyers overlook while comparing price per gigabyte.
#Why it matters more than it appears
Throughput is roughly concurrency divided by average response time. Doubling concurrency and halving latency have the same effect on how much work you complete. A cheap plan capped at a low concurrency can be slower in practice than a more expensive one with a high cap, at identical bandwidth cost.
#How limits are enforced
Behaviour when you exceed the cap varies and is worth testing deliberately: some providers queue the excess, some refuse the connection, and some return an error that looks like a target-side failure. Knowing which happens saves hours of misdirected debugging later.
#Set your own ceiling too
Even where a provider permits high concurrency, hammering one destination from many addresses at once is itself a detectable pattern and a good way to attract rate limiting. The provider’s limit is a maximum, not a target.