Unexpectedly slow number crunching

I've got a sub in my comms service module that constructs packets for sending via Asynchstreams.
I calculate a checksum for the data portion like this:

Code:

  Log("preparing checksum")
Do While cnt < sz-1
        oChk(0) = Bit.Xor(oChk(0),oData(cnt))
        cnt = cnt + 1
Loop               
Log("checksum done")  

I'm finding in some cases that this portion of code can take up to 20 seconds to run with a packet size of 4000 bytes. Why so slow, and is there any way I can...

Unexpectedly slow number crunching

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