Our customers have been complaining non-stop about how slow the new webmail interface is too.
Although it does seem the slowdown lies in initial load times that are entirely dependent upon the number of objects it has to index and paginate, once a folder is populated it does seem faster than the previous interface v15 and prior (granted "seems" is a relative word).
So we put it to the test and it did become clear that not all web browsers are created equal, especially in concerns with the new webmail interface. Some seem to handle AngularJS & CSS3 better than others. We tested on the same machine, same email account on the latest version of 8 different browsers and this is what we got:
||Time To Load
|Mozilla Firefox (Linux)
|Mozilla Firefox (Win)
For the new Smartermail interface having a modal page loading in 1.93 and 2.38 sec on Firefox (Linux) & Safari (Mac) it is definitely acceptable as that puts it into the Top 50% of fastest websites in 2017, and with Vivaldi it puts it at the Top 6% of websites, with all others but Edge and Firefox on Windows being in the Top 25% for speed, which is definitely a lot faster than the previous version all the way across the board. With Microsoft Edge and Mozilla Firefox on Windows the performance truly is abysmal compared to the old Smartermail webmail interface. (Remember, Firefox for Windows is still 32-bit and single-threaded which may account for the difference, especially when compared to Firefox for Linux which is 64-bit and multi-threaded.)
(Note: I didn't include Mobile Browsers as there would be too many variables causing them to be like comparing apples to oranges except when compared to those on the same OS on the same device...and besides I couldn't figure out how to enable DevTools on most Mobile Browsers.)
TBH, I can live with slower speeds initially as the priority at this point s/b quashing bugs as quickly as possible. I know performance is something that comes in time with optimization. I remember the switch from v7 to v8 in Smartermail, where even though the interface looked the same performance took a significant hit for several months before it was optimized better.