Currently switching between folders in Claws takes rather longer than I think it _should_ do. On my machine it takes ~5s to open an IMAP folder with ~100000 messages inside.

I have had a look at the code and there seem to be multiple causes of the issue:

 1. Folders are always rescanned over the network before being opened. This contributes several seconds of latency and download time.

 2. Folders (and their constituent summaryviews) are always rebuilt on opening rather than being cached.

 3. Building 100k GTK rows takes time (not much can be done here...).

 4. Extracting the information required for a summary row from the MsgInfo struct is expensive.

 5. Threading and sorting are also expensive.

With regards to 1., this seems like rather a design flaw, is there a good reason behind it? If not, it would make sense if scanning was completed in the background after the initial render of the folder.

Caching (in RAM) rendered folders as in 2. appears to be a larger architectural change but would make folder changes effectively instant no matter slow everything else is.

The code for 4. is relatively efficient, just the volume of messages and feature set requires that a lot of CPU time be spent on it. But a cache could be added (in the same vein as the imapcache) to reduce endless recalculations of unchanged messages.

I ran a profiler over the code for 5. and found two easy changes adding up to around a 10% CPU saving (of the total for loading a folder) - I can supply the patches for the changes. With some more substantial architectural changes I think another 5-10% at least could be saved. 

Any thoughts?