My volume response time between the 990 and Harmony One are very similar, but just a tad slower, than the original remote. I also have the inter-key delay set at 100ms. My other delay settings are:

Power Delay: 5000ms
Input Delay: 1000ms
Inter-Device Delay: 500ms

Some of the above are not default settings. If you have not already done it you could lower the inter-key delay to see what impact it has on the volume control. Also the descriptions of all these delays can be found using the "knowledge base" search command. Hit Troubleshoot and then two "none of the above" buttons. At that point it will give you an option to use the knowledge base and you can search using "input delay". And remember, Harmony has a terrific customer service staff and they will work with you over the phone to fine tune your new remote to meet your needs.