This can be due to a number of reasons - Too many markets in Guardian can consume resources and slow a machine down. Try restarting your computer in the first instance.
You may want to tighten up the connection tuner, you should try pulling the value down gradually to lower values. This will improve the API performance. Don't lower the value too much or else you may get no response at all from Betfair.
If you are trading Australian markets it is quite possible that the slowdown is related to the fact that your instructions are being routed to the Australian exchange. If you are not based in Austrialia this will obviously take much longer to send and receive instructions.