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Generally, there's no logical bandwidth cap on an IPSec tunnel. The tunnel might have a low default bandwidth setting, but that doesn't actually limit tunnel performance.
That said, there's much that can impact an IPSec tunnel's performance.
First, encryption can be very processor usage intensive, so performance will depend much on the supporting hardware.
Second, encryption consumes additional bandwidth, for its overhead, which can be a fairly large percentage for small packets.
Third, an IPSec tunnel can often lead to packet fragmentation, which consumes both additional bandwidth and CPU performance.
There are many performance measurement tools, some free, that you might use. Personally, on Windows hosts, I often use PCATTCP. It's a very simple tool; I find the UDP bandwidth generator often good for testing end-to-end bandwidth capacity. For example, I might set it to send 10 Mbps to the far side to confirm 5 Mbps gets there.
