My newest laptop has a Mobile Core i7, 6 GB RAM, and Windows 7 Ultimate and my ancient Eudora email client (with messages going back to the early 90s).
My copy of Eudora is the last ever version of 126.96.36.199 in paid mode (we go back to version 2.2 as paid users).
On this system there is a delay of up to 10 seconds any time you open a (sub-)window for the second time. This includes the mailbox listing / directories and individual messages. [Sub-window because this is is an old Multiple Document Interface (MDI) application where everything is inside the main application window.]
When the delay exceeds about 5 seconds you also get the (Not Responding) message added to the title bar.
Worse, if this system is running on the 90 watt adapter or batteries, it can take even longer and even hang Eudora completely. I have to run on the 120 watt adapter to make it open reliably.
Using the System Internals Process Monitor shows one CPU spiking during this delay.
The baffling thing is that the first time you open any given window, it takes under a second. But if you have closed a given mailbox directory, for instance, and a filter takes you to it again, you have this big wait.
NOTE: while I do have over 4.7 GB of mail data, Eudora 188.8.131.52 runs just fine with this same email archive on 32-bit Windows 7 Ultimate on an older Core-Duo (not Core-2) with 3 GB of RAM.
I have tried a lot of different things so far, including reducing the number of files in the "attach" folder and removing older messages from the inbox (currently holding about 500 messages).
I have several hundred filters but what I don't understand is why the performance is so much better on older slower hardware.
Another person in the company also has Eudora 184.108.40.206 (albeit without quite as large amount of archived email) and it runs on a Core2-Duo laptop with 4 GB of RAM and Windows 7 Ultimate 64-bit without these delays.