Both are mostly CPU bound. Compression seemed to have the biggest impact in tests done by @dgcom here: Duplicati 2 vs. Duplicacy 2.
Default compression level is 9 and going down to 1 cut about 36% off his backup time in one of the tests. An other test using level 0 cut that down about 45%. And the test with turning off encryption cut about 7% (for a total of 52% with level 0 compression and no encryption)
Of course these things could also depend on memory and disk speeds, or even network speeds if you end up bottlenecked there.