At least in the core functions the new ODS48 (Notes/Domino 8 database format) is not remarkably faster than ODS43 (Notes/Domino 6/7 database format).

I made a few tests with new blank databases (one for ODS43 and one for ODS48):

First I created an agent which creates 100000 documents (no fields).

The first test was to run the agent from the Notes 8 Basic Client:
notes:
ods43: 90.914s
ods48: 90.547s

The remaining tests were done using Mozilla Firefox using the Preview function in the Notes 8 Basic Client (ods48nc means ODS48 format with no design and document compression enabled):
http:

create 100k docs:
ods43:    Timer: 00108.78906s.
ods48:    Timer: 00089.81250s.
ods48nc: Timer: 00086.27002s.

change 100k docs (using documentcollection):
ods43:    Timer: 00024.68001s, 00024.50000s, 00023.56000s.
ods48:    Timer: 00025.16000s, 00025.02002s, 00024.13000s.
ods48nc: Timer: 00024.96997s, 00024.80005s, 00024.02002s.

change 100k docs (using view and getnextdocument):
ods43:    Timer: 00037.73999s, 00037.33008s, 00037.97998s.
ods48:    Timer: 00038.25000s, 00037.83997s, 00038.50000s.

change 100k docs (using view and getnthdocument):
ods43:    Timer: 00038.94995s.
ods48:    Timer: 00039.75000s.

read 100k docs:
ods43:    Timer: 00005.71997s, 00005.69995s, 00005.72003s.
ods48:    Timer: 00005.71002s, 00005.71997s, 00005.70001s.
ods48nc: Timer: 00005.72998s, 00005.72998s, 00005.75000s.



The speed advantage of ODS48 was better visible on a slower system. There I got times like:

create 100k docs:
ods43: 207s.
ods48: 194s.

read 100k docs:
ods43: 14s.
ods48: 11s.



As a further observation, I noticed that ODS48 with design and document compression enabled can be much slower when opening documents with the Notes Client.
This might be especially noticable on servers with lower CPU power and documents with big RichText fields.
The fastest database option in general might thus be: Domino 8.0.1 with ODS48 and no compression.