Are these Tag Value Changed events defined on the tags or are these Client Tag Change events (or Gateway Tag Change Events)? How many events do you have configured or how many tags are configured for the one event?
Also, take a look at the status page on the gateway for the device you are reading the tags from. What is your overload factor for the 350ms group? If its anything over 0% then you are not polling as fast as you think you are, and there is delay.
Are there any errors in the client logs? I'm assuming your change scripts are fairly simple, but to be able to rule them out, post an example one here, as preformatted text, see Wiki - how to post code on this forum.