There was no noticeable issue with my first wave of users, but I am now seeing a couple of lag-related issues in outlook.
All our outlook clients are on-prem, nobody accesses outlook externally. Our mailbox environment is pretty lean--we have 80 users but the total size of the MBDB is about 45GB
All user, shared, and resource Mailboxes are on EX16. Internal DNS and firewall is pointed at EX16. Our EX10 server is still in service but the only thing it is doing at this point is hosting the Public Folders until I run the migration process for that.
Issue one, that I'm less concerned about, is Outlook takes about five more seconds to load on average for a user who just has their own mailbox or nothing else. Before moving the mailbox, it was like five seconds or so to load for an average user, after migrating the mailbox to EX16 it is more like 10-15 seconds. It can be anywhere from 30-60 seconds if the user has full access to other mailboxes or resources. When Outlook loads, the lag occurs when the splash screen shows "loading profile," prior to the outlook icon appearing in the tray and prior to loading add-ins etc.
Thing two, I notice that dealing with additional mailboxes that I have full access to, there is a lag of up to 10 seconds if I navigate to a folder under the additional mailbox if I have not been in it for an extended period. Both my mailbox and the additional mailbox are on the EX16 server. When the mailboxeswere on EX10, the browsing was always instatnaneous unless the folder had thousands of items.
Hardware-wise the new server is a lot beefier. No hardware resources are close to maxing out (it's a physical host), plenty of RAM, not seeing any IO bottleneck. My expectation is that the new server would perform as good as or better than the old server... seems to be a mystery for now. At the end of the day, the increased lag is not life shattering but I'm concerend it could point to something being misconfigured that could be a worse problem some day.... Or maybe there's just more going in EX16 vs EX10 that would cause a little more lag via Outlook.