Internal and External MTU settings

We are investigation some slow network performance both internally and externally over vpn's.

Currently we have 19 Cisco 2800's switches into 2 Cisco core switches 3800's and 2 Cisco ASA 5520's.

The mtu on all devices is set to 1500.

If any internal or external (over the vpn) server / device is pinged with a mtu of 1500 the following message is displayed.

Packet needs to be fragmented but DF set

However if the mtu is lowered to 1400 then all pings are received correctly.

Could this be the cause of the general slowness ?

Any advice much appriciated and thankyou in advance.
Mongo PeckAsked:
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