Question: Is it possible to re-use the nxt_db directory when upgrading from 0.7.4 to 0.7.5 without downloading the whole blockchain again?
Yes, it is designed to be possible to upgrade without having to delete the nxt_db directory, that's the reason I bother with applying those SQL statements.
If too slow for Raspberries, do whichever is faster, upgrade or download from scratch.
Did I manage to improve the Raspberry performance with the last release?
once the blockchain is complete 0.7.4 is running quite smoothly at 1-3% cpu, and given lots of ram:
top
2040 pi 26 6 993m 385m 2768 S 1,0 88,0 61:54.85 java
3318 pi 20 0 4880 1324 976 R 1,0 0,3 0:01.18 top
I think also the blockchain download was faster with 0.7.4, but I can't really tell, because I did not monitor the dl all the time ...