The v16 webmail interface is improving with each minor version update...but it has been been slow progress...to SmarterTools credit they have been diligently putting out frequent updates addressing issues at a faster rate than usual until it stabilizes. To put things into perspective it took 13 years to get the webmail interface in v15 to where it is. It wasn't always as smooth and fast as v15 (fondly remembering the v4 days of old and the switch from v7 to v8 which was a bit of a mess). I doubt that v16 will take that long (or at least I certainly hope that it doesn't) but there is definitely still some ways to go.
Also I noticed other applications built on AngularJS and using Middle-ware having similar performance issues, so SmarterTools isn't the only ones experiencing this problem with a new web technology...especially on initial load times.
If it matters any, I've found that the webmail interface performs best in browsers such as Vivaldi, Brave, Opera, or the new Mozilla Firefox Quantum over the traditional Chrome, Safari, IE/Edge or old Firefox. 99% of the time everything I do in the webmail interface is almost instantaneous. I do have to hit CTRL+F5 more than once a day to resolve an occasional hung screen, but I don't frequently get the spinning wheel anymore (and if I do and CTRL+F5 doesn't immediately resolve it a REBUILD FOLDER in SmarterMail does). The only exception being displaying screens with > 250 items where it is exponentially laggy to the point of almost unusable...but even that has improved a bit recently.
Both I and our customers thoroughly hated the v16 interface throughout the past 6 months but it is starting to grow on us as the performance improves and is now at least tolerable with minimal hangs. In another 6 months (probably when v17 is due) it should hopefully be pretty stable.
But yeah, the new webmail UIX did need a little more time on the back burner before it was ready to be served, IMHO.