Does payee list size impact QM2015 performance?
Comments so far show QM2015 clearly inherited both good and bad features of QEM, QMac, QWindows versions (like children from their family tree). I truly love my QEM simplicity and ease of use. Very few added features in QM2015's first release are seducing me. The spectre of possibly degrading the peace of mind I enjoy with QEM currently keeps me in a holding pattern while I wait for the dust to settle -- or worse, Intuit stops supporting QEM simply to spur revenue from upgrades.
Experienced users (and I am one, from both Windows and Mac versions) know payee lists grow exponentially over time in any version or platform. That's especially annoying when I enter a frequently used payee slightly differently from purchase to purchase, or a financial institution does the same in a download (like appending a store number to a large merchant's name). It's a minor irritation when I enter transactions manually and see multiple automatic fill-ins. But more worrisome are reports of performance degradation from long payee lists.
My Question: How does QM2015 handle both growth and housekeeping? (In QEM, cleanup is tedious. A user has to view a payee list then combine similar ones. If I made an error long ago and deleted a "bad" name in a transaction, it's still listed. There is no "delete unused payees" button as there is with categories. But, conveniently, a failsafe warning does pop up when I try to delete a payee currently in a transaction.)

Comments
From my experience QMac2015 is very similar to QEM as to handling Payees.
There are two significant improvements introduced with QMac2015;
1) The Payee List has added a Status column and unused payees are noted. Lack of a note implies the payee is in use.
2) QEM added virtually every downloaded FI Payee to the list - even when the user had established a preferred name. QMac2015 does not do this and it keeps the list much cleaner.
FWIW, I have expended considerable time and effort in cleaning up the Payee List using the Merge and editing features - e.g., consolidating all the variations for 'Wal Mart' - various spellings, punctuations, store ID's, transaction date, etc. My list is very clean and now it stays that way.
Quicken user since 1991
In answer to your questions: in general, the # of payees is not the source of slowdowns. It can be if you have a huge amount of payees, but you would likely be seeing that in 2015 as well. I actually find performance of 2015 far better than Essentials. The default view is all dates for transaction view, and you can do incremental search very quickly.
That said, a few people have had serious slowdowns, but that appears data file specific (and likely corrupted).
The biggest complaints for speed I here right now are usually related to people who have a massive amount of securities and updating the prices takes a couple minutes. If you are using Essentials, then that won't be a concern to you.