Concerning the buffer size .. one is mentioned in samples and the other in milliseconds. Quite different ways to measure. Depending on the sample size (and maybe even the resolution being used) the two can correlate in very different ratios.

To solve the problem (just tips for getting on with the work and getting it done)

Try MME instead of ASIO - this can make biggest difference with least effort, regardless of 'why'. I see nothing you are doing that requires ASIO.

Instead of exporting/importing all those wave files, try highlighting the whole track and using Track - Consolidate Audio Region .. on each track you edited.
This effectively does the same thing.
You said you assembled the audio tracks from clipped bits and pieces (often done here). But now you have a track with a piece on the hard drive located at A, the next piece may be far away on the hard drive at spot B, meaning the drive has to jump all over from place to place to read the data in a stream. This jumping around can induce challenges that are removed if you consolidate the track into one contiguous chunk of data. Always save the file and reopen after doing this; it interweaves the data from all the tracks to make it all read even more smoothly. (This last part is theoretical but backed by experiences of myself and others documented here).

It may be a combination of CPU and RAM, but the above two things can be tried for no cost and have a decent chance of success.

I do not work here, but the benefits are still awesome
Make your sound your own!